Green Flu

"They're changing..."

- Bill

"This is some grim shit we got ourselves into..."

- 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.

Valve games typically contain stories that are expressed through exploring the world and interacting with its inhabitants rather than from watching lengthy cut-scenes or reading explanatory paragraphs 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.

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). One posibillity is that the infection might spread through boomer bile (like the achievement "OUTBREAK") or through the spitters 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 airborne strain of infection and cannot turn into Infected unless 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), 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. 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 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 existance 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 cut scene, 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 U.S. organization it 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 3 weeks after the first infection.

After the Infection consumed Fairfield the infection 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.

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.

Extent of Mutations
In terms of infection and mutation, the effects on people with the infection may vary, as aside from "Common Infected", Special Infected show extensive changes in mutation. A Common Infected is average size as they were human, have deathly pale skin, yellow eyes with no pupil or iris, have possible hair loss, and have weak body structure to the point that any blunt object, such as a guitar, can decapitate them with a single blow. While in those situations, Special Infected are intensely different. They have a much stronger structure than most Infected, and they all have some type of special ability.

It is speculated that their mutations are somehow linked to their lifestyles before infection. For example, a Hunter is the average size of a human, but is incredibly fast, is a high jumper, and crawls on all fours in easier efforts to pounce on victims. A Smoker is taller than the average human, possibly at 6 feet 7 inches, have an incredibly long tongue that can grow again after being cut, have a smoke of unknown origin that comes from their body after being killed, and have blemishes of some kind on their body. All Special Infected extremely differ from regular Infected, possibly showing that the infection has multiple strains that have effects on humans, and may attribute to their features (For example, the Witch and Spitter are both female, showing a difference). Another possibility is that before becoming Infected, certain hormones released by thought pattern, condition or emotion may have affected how they were changed. The most common would be fear or pain, which could be attributed to Common Infected. Rage could have lead to the Infected being becoming a Tank (although humans who substantially had more protein mass inside his/her body; in other words, muscular physiques could be good hosts for Tanks,) explaining its hostility towards the Survivors. A high level of physical activity, or good physical condition could lead to the production of a Hunter, from their Parkour attire. Depression possibly leading to a Witch which could explain her crying and listless wandering behavior, and why she lashes out at who she was disturbed by. The Boomer is often thought of as obese, but its stretched-out clothes indicate that Boomers were once of average build before swelling up into a Boomer. The Spitter seems to have either been irregularly tall or a mutated Boomer strain.

Another possibility is that the virus reacts with substances and hormones which were present in the body in significant concentrations at the time of initial infection. For example:
 * A Smoker could be caused by a human being who had been a heavy smoker in the past, causing the virus to react with cancer or tar in the victim's lungs, and thus mutating in a specific way. It could also be reacting to some sort of unknown fungus, given the smoky cloud of greenish-brown spores the Smoker gives off. It's quite possible the massive lumps on a Smoker's body are full of these spores, which is why they release a cloud of smoke upon death. A variant of this is similar: The infection reacted to the nicotine in a persons body, which results in a mutation that creates a smoker.
 * A Hunter mutation may arise in response to high levels of lactic acid in the muscles and bloodstream - a result of heavy exercise (such as fleeing from an Infected Horde). The specific mutation strain may be based on where the lactic acid concentrated when the subject was infected.
 * The Jockey's mutations may be similar to the Hunter's, paired with some form of dementia that either was already present or developed as a result of viral brain damage. Another possibility is any kind of hunchbacking or arthereosclerosis, which causes stunting and hunchback qualities that Jockey appears to posesses.
 * A Tank might be the result of abnormally high levels of Creatine or Human Growth Hormone - substances ingested when trying to build muscle mass. It could also be caused by a high level of testosterone, which in large amounts causes increased muscle and bone density as well as aggressive behavior and skin problems.
 * A Charger may be the result of someone being infected with the Tank gene, but unexpected reactions from the host (callousing and rashes, possibly) caused them to mutate differently. 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.
 * A Boomer might be the result of 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 'hijacked' as part of the infection's mutation.
 * The Spitter mutation 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 gangly limbs, sagging skin and awkward walk may be caused by the acid leaking into the rest of the body and breaking down connective tissues. It may have also 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.
 * In the case of a Witch, it could be the result of something which is normally present being unusually absent, such as Serotonin imbalance in the brain. Neurotransmitter imbalances are theorized to be a cause of Clinical Depression, this does not however explain the lack of male Witches or why she have such immense claws. Witches might also be caused by the virus interacting with the female sex hormone estrogen, which can cause wild mood swings, severe menstrual cramps and dementia in large amounts.

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 colours 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 easily disturbed they are, making it practically impossible to study them.

Contamination/ "Carriers"
Main Article: Carriers

In Left 4 Dead 2, it is revealed that people may be carriers. Carriers are immune to the virus, but still carry it in their systems. 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."

This leads to the possibility that people can carry the virus and not necessarily succumb to the effects. It is also implied that CEDA knew this and attempted to control the spreading of the virus. This is conveyed through graffiti in the same safe room.
 * "Why are they separating everyone?"

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

It is also later argued that carriers are of course immune, but still can pass on the Infection involuntarily. Another Survivor goes on to contradict that as well. It shows that people were not totally informed about the true behavior of the virus. CEDA had begun to separate people by use of colored wristbands to discern carriers from 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," proving that "immune" and "carrier" are one and the same (or at least that the military treats them as such as a precaution). Or it may also mean that they suspect civilians are unable to distinguish between immune and carrier, which would be likely in the case of a carrier traveling among immune people.

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. However, the sole concept of a "carrier" is rather odd, given the fact some graffiti state that the virus is not airborne, and supported by the Church Guy, who did not turn into an Infected form until bitten. Therefore, there would be no way for a carrier to spread the Infection unless he/she bit another person on purpose. It could also mean that the Infection could be spread through other means than biting, such as contact with infected blood or saliva. However, the Infected CEDA agents wear Hazmat suits that would prevent the attacker from breaking the skin, meaning that infection through biting may not be the only way of infection, while the suit itself would prevent airborne infection. Then again, in the Dead Center campaign, when seeing CEDA agents, Survivors may say something like "looks like these suits don't stop bites". On the other hand they somehow could have got Infected before putting on their Hazmat suits. Carriers could, alternatively, be people who are infected and will turn eventually, but have a stronger immune system, preventing them from turning for several days or possibly weeks.

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 un-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.

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. 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 was 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 that change times vary on strength and condition of the body. It could be possible that a very healthy body may take over the 4 days as stated, and an intensely weak body may take up to 5 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 to reach a major nervous center will vary.

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 bio hazard posters stuck to its glass 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.