Green Flu

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

- Rochelle

"Humanity's gonna bounce right back, you watch!"

- Louis

"Why couldn't I have been in Vegas when the infection hit? Or even Atlantic city? Instead I am trapped inside some hillbilly Tunnel of Love."

- Nick

The Green Flu is the name given to the unknown virus that has caused the creation of the Infected. The infection seems to be more like a rabies-like pathogen suggesting that CEDA's designation of the infection as a form of influenza is nothing more than a cover-up to avoid mass panic. Their lie has clearly failed as graffiti reading "Not A Flu" can be found throughout Left 4 Dead 2. The Infection in Left 4 Dead bears a resemblance to many of the more recent incarnations of zombie movies, video games, and books. 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 at all 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). 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 four Survivors are immune and cannot change into zombies. If spread by biting it is unusual that the Uncommon Common Infected CEDA Agent would get infected as they have more means of protection than other infected, the next in line would be the Riot Uncommon Infected as most of their body is covered except maybe the back of their neck, this could hint that the Infected may have a ability to know IF spread through biting how to spread it in the most vulnerable spot that is open. 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. Also, if the infection has regressed the Infected's mental abilities to a base desire to spread the virus (Or rather, attack anything on sight), and it was only contagious by biting or clawing, it would possibly explain why the Infected do not choose to make use of the weapons and ammunition found in the various safe houses and during the finale of each chapter. 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 simple zombies, 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 or temperament of the infected.

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 (which annoys him). 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." What this may mean is unknown, but 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. The Witch is also mentioned in some graffiti in a safe room, yet isn't named by the person that wrote it.

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. There is writing in a safe room within the No Mercy campaign that disregards the CEDA's safety step that tells Survivors to treat the Infection as though it were airborne. Also, being a strain of rabies, it's very probable that the Infection's primary form of spreading is through biting.

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. 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 England, 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 in Canada. The infection may not be able to thrive in cold weather, hence its inability to spread northwards. 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, New York City 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.

Contamination/ "Carriers"
In Left 4 Dead 2, it is revealed that people can be carriers. Carriers are immune to the virus, but still carry it in their systems. It is written in graffiti in the first safehouse 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 to 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, you encounter a dead (and infected) soldier with a radio, to which you signal the Military to rescue you. 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).

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".

Note: Many 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 up 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. However, graffiti nearby says that clean hands won't save you from voracious zombies.

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 or paranoid or spiteful. It is unknown whether any of the graffiti writers have any inside knowledge on the issue. Graffiti states that...

"Carriers = Zombies"

It turns out CEDA and the Military intend to save the 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.

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 CEDA, or more likely, the Military. There is also evidence in the form of graffiti in the "Hard Rain" campaign that known carriers were killed by uninfected civilians.

It is also noted that if you become infected by being bitten, it would make no sense to kill a "carrier" as they would not have the intention of biting someone. On the other hand, this does not rule out that it can't be spread other ways, as Virgil mentions his wife became infected, it is plausible that if he is a "carrier", he kissed his wife therefore infecting her unintentionally. (However, when calling Virgil, sometimes he will respond with, "It's been real quiet since my woman got bit" this seems to confirm the infection is spread by biting perhaps through saliva like rabies) This could bring the possibility that the survivors in the Left 4 Dead are carriers. This could also explain why in Swamp Fever, Nick and Ellis are arguing on how Nick shot the pilot. One of the survivors may have given him the virus and turned him to a Common Infected. The Military also thinks the Survivors of Left 4 Dead 2 are carriers hence The Parish Finale radio.



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. There 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. In addition, evidence suggests that infected people are infected some time after they have been bitten, allowing them to board planes and carry the infection around the globe. 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 the Church Guy was bitten an hour prior, while, nearing the end of said hour, manifesting mutations of a special infected.

Possible Original Victim
In the last parts of No Mercy there is a quarantine room on the fourth floor of Mercy Hospital around the corner from the elevator that is surrounded by yellow bio hazard tapes crossing the hallways and rooms near the quarantine room that also has several bio hazard posters stuck to the glass wall of the room. 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.

Trivia

 * On the upper floor of The Town's beginning safe room, there's a poster with a bed and the words, "Don't let the Infected bite." This may further suggest that the Infection is spread mainly through biting, although this could also be a play on the phrase "Don't let the bed bugs bite."
 * In the Hospital Stage of No Mercy, an X-Ray of a hand missing a thumb, much like the game's token image, can be found in various different rooms. If this person became Infected, they made it to the Apartments area of No Mercy as shown in the opening cinematic.
 * Through various conversations left as graffiti on the walls, it has been noted that other Survivors have noticed that the Infected can communicate to some level or at least utter words.
 * It is unknown if the Infection has sprung up in other countries around the world, or if it is isolated to the United States.
 * According to graffiti conversations between Survivors in the game, speculations of the origins of the virus range from the military, to divine punishment, to a hyped-up strain of rabies, to space aliens and Area 51.
 * According to various graffiti, signs and conversations, some people are naturally immune to the infection, such as the eight Survivors. This may imply that the Infection can only affect certain strands of DNA, which may suggest the Infection was originally a failed biological warfare project. This claim can be supported by safe room dialogue from Ellis, him stating that Keith once went camping where the military was testing bombs and biological weapons. Or the opposite may be true, in that the infection cannot, for unknown reasons, affect certain people's DNA, which also implies that a cure can be made. It is worth noting that natural immunity to viruses and other pathogens can occur in real life through a variety of ways; for example, a virus that gains access to cells through coupling with certain cell surface receptors might not be able to infect people with certain mutations of those receptor(s)—unless a chance mutation of the virus leads to a strain capable of infecting those previously "naturally immune." Of course, this is all only speculation among fans, based on various hints littered throughout the game. This has also raised a speculation that only certain blood types can be affected, and by the looks of things, and the amount of Survivors, this would mean that only the O blood type would be immune to the infection, that or most of the Immune Survivors were K.I.A.
 * In the originally intended sound files that would have been included at the conclusion of the No Mercy Campaign, the Chopper Pilot mentions being "Hit" as an infection method, displaying concern whether the Survivors were immune or not. He goes on to say that he thinks he may have been hit himself. Given the sounds following it suggest the pilot becoming an Infected (Confirmed with the DLC campaign Crash Course); the original method of infection may well have been simple contact. However, it is possible that the pilot did not actually want to mention the word bite, as he could have feared retaliation from the survivors.
 * Near the beginning of The Hotel level in Dead Center, a newspaper article refers to the Infection as 'Green Flu'.
 * However, graffiti later in the campaign contains such messages as 'Not A Flu', suggesting that the public was already suspicious of the nature of the infection even before it became widespread. It is likely that 'Green Flu' is just a catchy name used by the public, such as 'Swine Flu' is a term for H1N1.
 * The theory of the Infection being a mutated strain of rabies is backed up again in 'The Barns' level of Dark Carnival; in the barns, there are what appears to be dead cows, with the skin around their skulls scraped off, implying a brain sample; the most reliable way to test for rabies. It should also be noted that some cows were found in the Blood Harvest campaign and also appeared to have had brain samples taken. And even again it is backed up in the campaign 'The Swamp' cows that have the skin on their head scraped are seen in a pile with dead bodies of people from The Swamp in a fire pit with wire around it.
 * The Midnight Riders state that the Green Flu will not stop their tour of the American south. This seemed to be proved wrong in one of the safe rooms in Dark Carnival, the graffiti states: " R.I.P. Midnight Riders ". This is corrected by another Survivor stating that they were rescued by a chopper.