Green Flu

"They're changing."

- Bill

"Looks like CEDA should have been telling us to do more than wash our hands."

- Rochelle The Green Flu , commonly referred to simply as The Infection, is the name given to an unknown virus that has caused most humans who come in contact with it to change into homicidal, zombie-like beings known as the Infected. It seems to be a rabies-like pathogen, and CEDA's designation of the virus as a form of influenza appears to be nothing more than a cover-up to avoid mass panic. Graffiti reading "Not a Flu" can be found throughout Left 4 Dead 2, suggesting that the deception has failed. Promotional materials from multiple sources state that the virus is a mutated strain of the rabies virus. There has been much speculation as to the cause of the infection, or whether or not its origin will even have any relevance in the plot of the game. In terms of containment, The Green Flu is likely a biohazard level 4 organism.

Valve games typically contain stories that are expressed through exploring the world and interacting with its inhabitants rather than from watching cutscenes or reading explanations out of the game manuals. This seems to be the case in Left 4 Dead as well. Graffiti covers many walls of the game, often containing messages to separated loved ones, foreboding messages of warning, and, most interestingly, past Survivors' own explanations for the infection, varying from government conspiracy to alien plot to an act of God.

In The Sacrifice it is explained that the immunity is passed down through the father's genes. The most common theory to explain this is that the gene that makes people "resistant" is a recessive allele on the X chromosome. In Zoey's case, as well as Rochelle's and any other female survivor's, she must have gotten an X chromosome with the recessive gene from both her mother AND her father. Zoey's mother was not resistant, meaning that she only had one X chromosome with the gene (the dominant gene on her other X chromosome would have override the recessive one). As explained, her father was resistant, making his death an avoidable tragedy. This would also explain why there are more male than female survivors, as a male survivor would only need one copy of the "resistant" gene found on his X chromosome (his Y chromosome would neither have nor need this), while any female would need two copies - one on each X chromosome. In fact, the number of immune males would be the square of the number of immune females, as per basic Punnett squares. If the sex-makeup of the two survivor groups is representative, then we would expect one in three males, and one in nine females in the pre-outbreak population to be immune to the infection - a net 22% of the population.

Means of Infection
The intro movie to the game begins with the cryptic statement "2 WEEKS AFTER FIRST INFECTION", implying that a single individual was once Infected with an unknown virus of some sort, and the virus spread from there. What's not immediately clear, however, is the means of spreading the infection. The Infected don't seem necessarily ready to bite players, as other more "classical" zombie viruses spread (though the "Church Guy" mentions he was bitten and shortly thereafter turned, and there is a dead man on the first map on The Passing campaign at the second floor of the store, slouched in a chair with what seems to be a bite mark on the right side of his neck. Also at the beginning of the SWAMP FEVER campaign a conversation between Nick and Ellis is heard explaining that the pilot of the helicopter 'must have been bit before he picked us up'). One possibility is that the infection might spread through Boomer bile (like the achievement "OUTBREAK") or through the Spitter's spit. During one of several possible dialogues with the "Church Guy", Bill says "Son, we're immune, we're tired, and there's Infected in the damn woods, now cut out the shit and let us in!", providing evidence that the Survivors are immune to the infection and cannot turn into Infected even if bitten. Typically, for an infection to spread to victims wearing personal protective equipment (PPE), like the CEDA agent, a breach incurs exposure in one of two ways and also indicates the route of infection. The first breach is the more obvious, which is through the suit via biting (Parenteral Transmission) or needlestick (accidental inoculation), indicating the green flu pathogen resides in the blood and/or saliva of the infected person (a rabies-like trait). The second involves a breach in the exterior air supply to the victim which indicates an airborne or aerosol strain of the virus is also at work. This was made more plausible in The Sacrifice comic, where it is mentioned that the virus is "sometimes airborne, sometimes not". However, this suggestion is atypical of rabies and is more common in influenza strains. From evidence presented in the game, biting and fluid exchange (i.e. blood, saliva, and vomit ingestion) is the primary form of transmission. A separate airborne strain of the virus could explain the speed at which infection occurred. If a victim is infected with both bite and airborne strains, it could also explain the fast mutation of the virus in victims and the development of Special Infected. The Sacrifice tells us now that despite the name and tendency to spread through bites, Green Flu is not rabies or flu but rather a new virus that mutates on a daily basis.

The Infected appear to favor the immediate death of the Survivors rather than infection through biting or some other method, which might mean they display some ability to detect immunity or still have some small sense of self-preservation. It could also be that the Infected have lost their traces of humanity and run on animal instinct. It is not explained under which factors certain victims become Common Infected, while others become Special Infected, though it is possible that there are different strains of the virus that can only survive inside certain host bodies, or it depends on the mental state, habits of the host pre-infection, or temperament of the Infected. However, the prospect of having multiple strains of a particular virus onset so quickly and naturally is not very plausible for today's scientific and medical standards. Evolutionary trends show it takes hundreds to thousands of years for recombinant, or similar, strains of a particular virus to become so populous that they can be considered a completely separate strain. If indeed there are different strains of the virus at work and cause the onset of special infection symptoms, then it is more likely that the virus was engineered. It should be noted that "engineered" is a vague explanation and could mean anything from a biological weapon to an experimental vaccination gone bad. It should also be noted that random mutations of a virus and the existence of different strains of a single virus are two separate scenarios. The idea of an engineered virus only applies to the latter.

Immediately after the aforementioned intro card for the opening cutscene, Bill is seen discovering a viscous, bright green substance in or on (it's difficult to discern) a dead (and presumably Infected) body. He mentions that he's never seen anything like it before, but it doesn't seem to hurt him in any immediate way, and neither does smearing it on Francis' vest. This is most likely Boomer bile (a Boomer corpse is seen in the background if observed closely enough). The substance was said to reek, and Bill claims that the liquid was a sign that the Infected were "changing." While this provides no definitive explanation, it may imply that the Infected are still in the process of mutating, and that all the changes the virus will cause to its host is as of yet unknown. This could mean that the Special Infected were just starting to develop and this was the Survivors first encounter with one. This is backed up by how Zoey approaches what she assumes is a Survivor crying, despite the obvious signs of her being a Witch, implying that she has never encountered one before. However, it seems that Bill does recognize the Witch, ordering Zoey to shut off her light and back away, or alternatively, may have realized the Witch as infected due to her being illuminated by the lightning.

CEDA posters in the airport suggest that the Infection is related to livestock, and later, towards the end of the Blood Harvest campaign, the player will encounter several deceased cows, neatly stacked atop one another. The cows typically have the skin around their skull missing, indicating that rabies tests were performed on the animals. Cows in the Barns level of Dark Carnival and in the Swamp Town level of Swamp Fever also have the skin to their heads removed, and upon seeing the dead cow pile in the Swamp Town, Rochelle sometimes mentions that the news claimed the virus spread through livestock, and believes the natives of the town, determined to survive the Infection on their own, killed their own livestock to prevent an outbreak. All the dead cows might be a reference to real world Mad Cow Disease, amongst all the other inspiration from real world diseases. There is also writing in a safe room within the No Mercy campaign and seen before the River Tour in The Passing that disregards the CEDA's safety step that tells Survivors to treat the Infection as though it were airborne.

Spread of the Infection


The map proves that the epicenter of the Infection was in Pennsylvania, where it quickly spread across the eastern United States. This provides the explanation for why Fairfield's cityscape was ravaged in the first game. Most of the cities in the region covered have fallen to the Infection, showing how fast-spreading it is. The map indicates that despite originating in Pennsylvania, the Infection might not have spread north, as there are no "X's" in New York, meaning the region may not be overrun. The map also seems to indicate that the Infection hasn't spread to Canada yet. However, it is possible that because of the location of the Infection, the military and CEDA do not have any information about that region, or that because CEDA is a US organization and has no data on the Infection beyond the United States' border. It should be noted, however, that larger cities such as Toronto, Montreal, and Ottawa fall within the first circle indicating the spread of the Infection, suggesting that they too are besieged. Some of the larger cities in the Infected area have not been crossed out, indicating that the cities may still be viable extraction sites that have been locked down by the military. However, Philadelphia is in the Infection epicenter, so it is likely it was overrun soon after the Infection started, and the nation's capital, Washington DC, was apparently besieged, as its location is crossed out on the map. The only viable evacuation point left in the South Atlantic region is New Orleans, Louisiana, which itself is besieged three weeks after the first Infection.

After the Infection consumed Fairfield, it spread to the southern United States and lead to more and more groups of Survivors to arise to try and get to safe zones created by the government, which, as seen in the Left 4 Dead 2 trailer, started bombing cities to stop the Infection from spreading. A CEDA evacuation poster in Crash Course seems to imply the Infection has spread all over the United States, as there are safe zones indicated in the Northeast, Midwest, South and West, of which the West, Midwest and Northeast are crossed out, meaning they were besieged, possibly hinting that the Infection may be climate-based.

The Military held safe zones/evacuation centers across the country, with fortifications in small towns surrounding Philadelphia, such as Riverside and Echo base, with several evacuation efforts throughout population centers, but they were forced to pull back by the time the survivors get there. In the South, Atlanta and New Orleans were both fortified, but there are reports that Atlanta was either overrun or the Military pulled out, regardless New Orleans is the last known evacuation center by the time the survivors get there, but was overrun, with the Military resorting to bombing the area. On both the map in Dead Center and Crash Course, every evacuation center is crossed out, the last possible area still uninfected would be on the coast of Washington State (the evacuation center in California may have been overrun due to the fact that it has been crossed out), as the map in Crash Course had an orange line leading to the area, but the section of the map that showed the evacuation center itself was torn off.

Because the map only covered the South Atlantic area, the status of the other evacuation areas are still unclear, though going on the prior evidence the other safe zones are either overrun or under heavy zombie attack.

Rural areas may have been the first to become infected. Animals were probably the first to contract the disease (other than the first initial human infection at Mercy Hospital), which could explain the piles of dead cattle on rural campaigns such as Blood Harvest. Alternatively, the cows may have been killed due to human paranoia, similar to Egypt culling all of its pig population during the real life event 2009 flu pandemic (also known as swine flu) for much the same reasons. There are also posters at Metro International Airport, which ask people to wear masks, and tell all who have been in contact with livestock to tell airport staff immediately.

Spread by days
Little is known of the exact dates and time is rarely given, but according to the Midnight Riders tour list and posts on their website, the events of Dark Carnival are probably set on October 8-10, 2009.


 * Day 2: At this point it seems most people still think the infection is just another flu and was not that harmful. However, it also seems this is the day Philadelphia was overrun.
 * Day 7 (assumed): Last time Millhaven finds any other Survivors so most of the surounding area has probably been infected by now.
 * Day 11 (assumed): Last time Millhaven hears anything from command.
 * Day 14: By this time most of Pennsylvania is overrun, and the infected start mutating into the "Special Infected".
 * Day 16 (assumed): The Military bombs several airports to slow down the spread of the infection.
 * Day 19 (assumed): Millhaven Millitary outpost is overrun due to sounding an alarm.
 * Day 20 (assumed): By this time and probably earlier, The Infection has reached Rayford, Georgia.
 * Day 21: The infection reaches Savannah, Georgia. This is also assumed to be the day such mutations as the Charger, Jockey, and Spitter started appearing in the southern states.
 * October 8, 2009: The Infection reaches Griffin, Georgia so the Midnight Riders have to cancel a concert and are picked up by a helicopter.
 * Day 22-24: Somewhere around this time the Military took over all of CEDA's outposts and operations due to several failures of containing the infection.
 * Day 24 (assumed): New Orleans is evacuated because of the infected population, and Veterans Memorial Bridge is bombed to slow down the infection.

Extent of Mutations
In terms of the Infection and mutation, the effects on the general population vary. The average Infected person results in becoming a Common Infected. The Common Infected have deathly pale skin, yellow eyes with no pupil or iris, possible hair loss, and a weakened or weakening body structure to the point that a guitar can decapitate them with a single blow. The change in eye structure seems to have altered an infected person's vision. No matter how far along in Infection is, non-Infected appear as monsters in a Common Infected's eyes (but not the eyes of a Special infected, as shown in-game). This can explain the hostile behavior Infected have towards Survivors in order to defend themselves, and the fact that Special Infected (besides the Tank) don't attack head on, and wait for their prey. There appears to be little change in body sizes, although it can be assumed that there is some sort of weight loss due to starvation. If there is starvation occurring, then this is indicative of a mutated hypothalamus in the brain. Further, an Infected person would not feel hunger, thirst, feelings of fatigue, nor have a regulated body temperature. Graffiti found in Dead Air backs up this statement.

In rare cases, individuals show extensive physical and mental changes after infection. In such situations, these Special Infected are physically stronger than Common Infected and have experienced some sort of side-effect or mutation that results in a unique ability. The symptomatic mutations are very distinct per Special Infected type, indicating a person-to-person variation in reaction to infection. There are an infinite number of speculations that can be made about the exact cause of mutations for the Special Infected. Valve has been somewhat evasive when it comes to providing epidemiological explanations or evidence for the infection and development of Special Infected. Below are several possible explanations and contributing factors for the development of Special Infected.

Gender-Linked Mutations: From evidence presented thus far, the Special Infected mutations are predominantly gender-linked. The Witch and Spitter are both female with no male equivalent being observed (yet). The Jockey, Hunter, Smoker, Charger, and Tank are all [presumably] male with no female equivalent. The only exception is the Boomer, who has been observed as both male and female. Besides anatomical differences, the most notable difference between males and females is sex hormones (Testosterone and Estrogen/Progesterone). If there is a partial gender-link to the mutations, sex hormones are likely to blame. The word “partial” was used because sex hormones could not be the sole reason for mutation and there would have to be another contributing factor.

Multiple Virus Strains: The extreme differences between Special and Common Infected could hint at the existence of multiple strains of the virus. A simple example would be a Tank-specific strain that causes the metamorphosis of a human into a Tank. This idea is problematic because it is atypical of viruses to arise so quickly and develop multiple strains within days. Not to mention viral genomes are typically too small and limited to generate new “breeds” of organisms after infection. It’s not outside the realm of possibility for this to occur naturally, but very unlikely. The extremely low frequency of special infected is another problem with this idea. According to the pie chart below, roughly 1/3 of the infected population becomes a special infected. Within that third, there are 7 different special infected subtypes. Assuming that each subtype is a separate strain, the average infection rate for the special strains is ~4.8% (33.333% divided by 7). Multiple virus strains of the same viral species typically have comparable infection rates. In this case, you have 7 strains infecting at a rate of less than 5% and 1 strain infecting at a rate of 66%. To put it simply, if there is multiple virus strains then in theory there should be more special infected. On the other hand, there are a couple of possibilities that could explain the low numbers. The other strains could have become prevalent after the majority of the population was already infected. If a good portion of the population is already infected, then the rate of infection for new strains would be much less. Another reason is that the special infected may not be able to spread their strain as readily. For instance, the only contact between a human and a charger is when the human is charged and pummeled into the ground. Such contact, incredibly, would not be considered significant enough for disease transmission. Odd infection trends and rapid viral mutation could be explained using more radical ideas in order to make them more plausible. Multiple viral strains could have been artificially engineered by an ignorant science facility or government agency and caused an accidental or purposeful outbreak, whichever the case may be. It should be noted that there has been no evidence that the virus was or wasn’t engineered.

Genetic/Biochemical Influences: Since each member of a Special Infected type has virtually identical phenotypes (physical appearance and mutations), it’s likely that the extensive mutations arise from common factors. While the exact factors remain a mystery, it’s feasible that such factors are related to genetics and/or biochemical by-products (supplements, vitamins, foreign substances, gene-linked proteins). All genetic-related abnormalities are related to some biochemical component produced by an uncommon gene. A presence of, or lack thereof, an influential biochemical would have to be occurring for a gene-specific cause to be a factor of mutations. Scientifically, it’s not very sensible to think that someone with a genetic abnormality would be a viable candidate for becoming a Special Infected. The alternative to genetic factors is a common adverse biochemical reaction between the infection and various substances already present in a body. Examples include anything from medication to supplements to by-products of other ailments. Based on evidence presented in the series, this is the most popular explanation for the extensive mutations. Examples for each Special Infected are listed below:
 * Smoker Mutations may occur to someone who is/was a heavy smoker. Higher levels of cancer or tar in the victim's lungs could inhibit complete exhalation and fill the respiratory track with “smoke.” Higher concentrations of the “smoke” in the body may facilitate further mutations. There also could be a reaction to a fungus strain, given the smoky cloud of greenish-brown spores the Smoker gives off. The massive lumps on a Smoker's body could be full of these spores, which release a cloud of smoke upon death.
 * Hunter Mutations may occur due to interactions with elevated levels of lactic acid in the muscles and bloodstream—a result of heavy exercise (presumably from a continuous exercise-heavy lifestyle, such as parkour; fleeing from an Infected Horde is rather heavy exercise, but this alone would produce a large number of Hunters).
 * Jockey Mutations may have resulted from someone who was confined to a wheel chair. The obvious regenerative properties (mutations and growths) of the Infection could indicate a restoration of spinal cord function, giving the infected person the ability to walk again. This would explain the hunchback appearance and the emotional mania. A separate speculation suggests that the Jockey's mutations may be comparable to the Hunter's, paired with some form of dementia that either was already present or developed as a result of viral brain damage.
 * Tank Mutations are possibly related to body building supplements. Abnormally high levels of Creatine, Human Growth Hormone, and anabolic steroids (possibly produced naturally from some damage by the infection) could interact with the virus to promote tumor-like growth of muscle cells. If left uncontrolled, muscle mass would increase exponentially to the point where complex muscle movements are lost or limited and only basic movements (shuffling, climbing, thrashing, slow throwing, etc.) are possible.
 * Charger Mutations are similar to the Tank and could mean that the Charger is a hybrid version of the Tank. Some steroids are used as an anti-inflammatory remedy for a rash or growth on one side of the body. An abnormal interaction with a localized and smaller concentration of steroids could have created the Charger. It might also be a reaction similar to gigantism (where excessive amounts of growth hormone is released, resulting in thickened bones and asymmetrical body structures), given that the Charger's facial features and overall body shape is similar to that of someone with gigantism.
 * Boomer Mutations probably relate to abnormally-high levels of fatty tissue or cholesterol in the bloodstream, which would also explain the large amounts of bile they produce. Bile is created to aid in the breakdown of lipids (fat). The human body's natural bile production would have been assimilated as part of the Infection's mutation. This would explain why, when Boomers burst, their torsos appear to be largely hollow. The space previously occupied by fatty tissue had been broken down and converted into a single, enormous sac for containing and producing bile.
 * Spitter Mutations may have resulted in infecting a human subject with a helicobacter pylori infection (bacterial ulcer). Cells previously infected by the bacterial agent may have been consumed by the virus and caused it to mutate. Another take suggests that the mutations might be similar to the Boomer, except the hormone gastrin (responsible for making stomach acid) is hijacked, resulting in Spitters being able to create large amounts of acid. The Spitter's sagging skin and awkward walk may be caused by the acid leaking into other bodily cavities and breaking down tissues.
 * Witch Mutations seem related to an abnormal mental state which could indicate a predisposed mental disorder, like Autism, ADHD, Serotonin imbalance in the brain, etc. Neurotransmitter imbalances are theorized to be a cause of Clinical Depression. Psychological trauma could also be a factor, such as with the "bride Witch", who may have suffered the mental stress needed to become a Witch when the Infected attacked her wedding. Since the Witch is one of the most notable gender-linked examples of the Special Infected, it is likely that there is also some sort of influence from female sex hormones. It might also be a reaction for anorexia, as all Witches are thin women, and attracted by sugar smell possibly due to lack of carbohydrates.

The poster found in one of the rooms in The Hotel shows a pie chart indicating the estimated percentages of each strain of Infection. The colors are assumed to be:
 * Common Infected - Red (seen on board)
 * Charger - Green (seen on board)
 * Spitter - Orange (seen on board)
 * Tank - Pink (seen on board, also due to relative smallness)
 * Boomer - Olive (large line pointing to it)
 * Hunter - Yellow (most common to play as in Versus in the original)
 * Smoker - Blue (is about half of the yellow slice, roughly the same size as the Boomer)
 * Jockey - Purple (Most feasible slice left)
 * The Witch is likely to be the slice not included, due to the relatively small number of them, as few are found in campaigns, as well as how deadly and easily disturbed they are, making it practically impossible for humans to study them safely.

Contamination/ "Carriers"
Main Article: Carriers

In Left 4 Dead 2, the potential existence of Carriers is referenced. Carriers are asymptomatic survivors that look perfectly healthy but "carry" the virus in their system. Therefore, a carrier can unintentionally transmit the virus to normal uninfected individuals making carriers a threat to public safety. It is written in graffiti in the first Safe House in the campaign "The Parish", and revealed by the military over radio in the finale of the same campaign.
 * "I've been here a week, and they're shipping out people who JUST got here..."

Is written on the wall, to which another Survivor responds:
 * "Those are Carriers."

Since grafitti is typically hearsay, this implies that carriers are rumored to be real and suggests that there was at least one instance where normal healthy people were becoming infected by being in the same vicinity of other healthy-looking individuals. Other graffiti in the same safe room suggest that CEDA and/or the military acknowledge the existence of carriers and were taking steps to contain further infection:
 * "Why are they separating everyone?"

Another Survivor writes:
 * "Some people are Carriers."

More graffiti argue that Carriers are of course immune, but still can pass on the Infection involuntarily. Another Survivor goes on to contradict that as well. The existence of contradictions in the third person conversations show that people were not totally informed about the true behavior of the virus. According to some of the posters found in the Parish, CEDA had begun to separate people by using colored wristbands to discern Carriers from the uninfected population. Penalty for failure to comply with the regulations regarding the wristbands were made clear on a US Military poster stating that use of lethal force was authorized on those seen without color indicating wrist wear. Some graffiti shows an extreme dislike for Carriers: "The only good Carrier is a dead Carrier".

Later on in the campaign finale, a dead soldier with a radio is encountered, with which the military is signaled for rescue. The voice on the other end, going by the code name "Papa Gator", asks the caller if he/she is immune, to which the team affirms. "Papa Gator" then orders the last helicopter to await your arrival, asking if the pilot is "equipped for Carriers." This confirms carriers do exist and that the military are equipping their rescue vehicles to support safe and quarantined transport. It also shows that the military is treating any asymptomatic survivor who has come into contact with the infected as a potential carrier. Such treatment would be necessary since it is nearly impossible to distinguish between survivors who are immune and Carrier without some sort diagnosis test.

It may have very well been popular opinion that the Carriers could in fact spread the disease, and that they were on their way to be terminated. But since carriers are normal healthy-looking individuals, it's highly unlikely that a carrier was biting others and transmitting the disease. This brings into question how exactly the disease is able to be transmitted by other means besides biting (parenterneal transmission). Other routes of transmission include saliva, blood, mucus (from sneezing/coughing), and airborne. While some graffiti state that the virus is not airborne, the fact that the CEDA agents were wearing highly protective hazmat suits suggest otherwise. Such suits are primarily designed to prevent airborne transmission and the fact that CEDA was even using such precautions correctly asserts that the existence of an airborne strain remains unconfirmed. At the same time, there is no confirmed route of disease transmission. In the Sacrifice comic it is revealed that the virus mutates continually, including going from being airborne to only being transmitted by contact.

However, it is worth noting that any viral organisms can survive on surfaces for a number of days should an Infected individual touch something with their hands. Everything from door handles to cooking utensils are fair game for the bug to live upon, and will be transferred to anyone who would come into contact with it after wards. It is for this reason that even in the administration areas of hospitals people are required to have hand sterilization dispensers for anyone who has been handling patient records, which are a prime source of infectious contact from patients and the doctors who treat them. All that is needed after that is for their hands to come in contact with food, their mouth, their eyes or any open wound. There are also CEDA posters that say "Clean Hands Save Lives!" around the places of quarantine which supports this theory.

If the Infection is carried by body fluids, it is possible that some or all carriers may in fact be regular immunes who have had heavy contact with the Infected. The Survivors are regularly splattered with blood, brains, and various other fluids and parts of the Infected during combat, and it is possible that some or all of these carry the virus in a form that the non-immune are capable of catching. A Survivor carrying the Infection in this fashion would remain a potential hazard until a full disinfection procedure could be completed, which would be difficult for the military to perform under the circumstances present at the checkpoints. This is likely originally planned, as The Waterfront and The Park were originally one level, and had a decontamination chamber (complete with shower sprays) where the first crescendo of the campaign was.

However, other graffiti contests the Carrier theory, calling it a lie. It is unknown why the opinions differ. The deniers could be in a state of disbelief, or those spreading the theory could just be speculating, paranoid, or spiteful. They may also know that CEDA's statement that the Infection was a form of influenza was a lie told to keep the population under control, and suspect the military of using the idea of carriers to pull the wool over their eyes in a similar fashion. It is unknown whether any of the graffiti writers have any inside knowledge on the issue. Graffiti states that...

"Carriers = Zombies"

If the Infected are able to sense immunity, and will attack to kill and not to spread the virus, then it is also reasonable to assume they could detect if a person is a Carrier. The Infected can obviously tell which people also have the virus (disregarding random fights amongst themselves occasionally), and it would be strange for them to attack someone who is capable of spreading the virus, making another possible point against the true existence of Carriers.

It appears that the military intend to save Carriers, as they are immune to the virus - as shown in the finale of "The Parish" campaign, where it would have been much simpler to leave the Survivors to die. Unless they wanted a live Carrier to inspect (or dissect).

Groups of corpses can be found (second safe house of The Parish) which have not been Infected but instead suffered gunshot wounds. It is possible the victims were "Carriers" who were killed by the military. There is also evidence in the form of graffiti in the "Hard Rain" campaign that known Carriers were killed by non-Infected civilians. In The Parish, right before the Bus Depot when the bodies are discovered, the Survivors realize that they were killed by military. Nick also comments in the finale of "The Parish" campaign that the military will probably line them up to a wall and shoot them all if they make it across the bridge. However, the military could have also simply shot at the crowds of Survivors if there was widespread panic, in order to quickly regain control of the situation. Though, in such a situation, it is more likely that non-lethal ammo would've been used, since actual killing could potentially make the situation harder to control, this is also evidenced by infected seen in the same campaign with riot gear and nightsticks, which are meant to be non-lethal.

Time it takes to turn
Though there has been no official confirmation by Valve on how long it takes for an Infected person to turn, there is a conversation on the wall of the Rooftop Finale's starting safe house in graffiti. A number of unnamed Survivors wrote down the time they think it takes to change. One had seen someone turn in four days, and more people come along to "correct" each other until the last person writes that they saw someone turn in 5 minutes. It's possible that the time it takes varies from person to person, as the Church Guy claims to have been bitten an hour before the Survivors make it to his safe room. Further to point, in The Sacrifice comic, Zoey's mother turned mere minutes, if not seconds, after being bitten. However, this may mean that each strain of the virus does vary in its time to completely turn Infected. If so, then the reason for varied times is probably the result of mutation of the virus. It may also be dependent on the immune system of the host, as the Survivors prove that it is possible to be completely immune to the virus. Mutations, however, seem to take hold almost immediately, as Church Guy was bitten an hour prior. Nearing the end of said hour, he manifested mutations of a Special Infected. It is also possible that, since we never see Church Guy as a human, he had already begun undergoing most of the mutations but his questionable sanity and apparent denial prevented him from noticing them (or allowed him to disregard them) until the Infection finally affected his mind.

It is possible that the amount of stress (psychological, emotional, and/or physical) may play a role in how long it takes for a Survivor to become an Infected. For example, the Church Guy did not become Infected until becoming startled by the Survivors, which could have caused the Infection to rapidly mutate him. As they arrived at the church, he is overheard muttering in an attempt to convince himself that he is immune, which could very well have been slowing his Infection rate. An adrenaline surge (such as one triggered by an intense fear or stress reaction) accelerates the pulse, converts fat cells into fatty acid, and constricts the surface blood vessels, redirecting blood to the muscles and deep tissues of the body. Reactions like this could play a significant part in accelerating a person's transformation into an Infected. It is possible that change times vary on strength and condition of the body. It could be possible that a very healthy body may take over the four days as stated, and an intensely weak body may take up to five minutes. If this theory is correct, along with the stress theory, it will have multiple effects in changing, and possibly in the extent of mutations. The Church Guy obviously was a worried man, hanging onto what remained of his sanity, but his body strength and condition is variable, as he is never met face to face. If this applies, the Infection and its outcomes are very flexible, making it hard to determine a person before they are overrun with the Infection, and how they will turn out.

Another speculative hypothesis is that, seeing as the virus may be a mutation or sister strain of the rabies virus, the point of entry to the body may also have some sway on a victim becoming a full blown "Infected". The reasoning behind this is linked with how long the virus takes to reach the brain from the point of infection; that is to say, it will take longer for the virus to manifest itself if the victim is bitten on the foot, perchance, than it will if the victim was bitten on the face or neck, as the distance the virus needs to travel in the bloodstream (or the much slower lymphatic system) to reach a major nervous center will vary. Other factors may contribute to this, such as if a survivor was bitten on a blood vessel, in which case the blood would be moving towards the system, as opposed to an artery, in which case it'd be moving away from the system through an open wound, thus giving the survivor more or less time before turning depending on which, if either was penetrated. Also if the infection does not travel through the bloodstream, but instead through the lymphatic system (like a snake bite) - then it's spread would be slow and dependent on physical exertion. Thus keeping the patient subdued and the wound-site immobilized in a properly applied pressure bandage may slow the progress.

In the Sacrifice comic part 2, Zoey's mother is bitten and seems to transform in a matter of seconds, while one of the Military personnel has transformed into a Boomer in a matter of minutes after the Infected start attacking the base.

Another possibility is that Special Infected might turn when they hear a high-pitched noise. Just like when a common infected is attracted to it. An example of this would be the church guy in Death Toll he was apparently bitten an hour before the Survivors found him, but he does not turn until he himself rings a church bell. Other examples of people who possibly turned due to noise would be the army-Smoker on page 87 and 88 of the sacrifice comic, and the army-Boomer on page 81. All of these three examples turns during some loud noise. However, there is no proof of the noise actually causing it.

Possible Original Victim
In the last parts of No Mercy on the fourth floor of Mercy Hospital, around the corner from the elevator surrounded by yellow bio hazard tapes, there are multiple hallways and rooms. One of these rooms is made of glass. This quarantine room has several biohazard posters stuck to its walls. Inside is a corpse of a man lying face down in a hospital gown with blood around his mouth. This seems to hint that he was at least one of the first, if not the first, in the area to be Infected, maybe even making him the original victim of the Infection. However, this may simply be an Infected person that doctors believed they could save. Once they failed to do so, it may have led to the inevitable spread of the Infection within Mercy Hospital.

Also noteworthy is the fact that bags of IV fluid (which looks like blood) on racks near beds at Mercy Hospital have the word CONTAMINATED stamped in bold red print across their label. These contaminated IV bags can also be found in CEDA medical tents in Dead Center, indicating CEDA's concern over a blood borne pathogen.

This may be most likely, as the Infection did start in Pennsylvania, the original setting of the first game.

It is also quite possible that the Infection was spread through Carriers, which would mean infected could move around large numbers of non infected humans, then pass it on unintentionally.