Green Flu

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

- Rochelle

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

- Louis

The Green Flu (also known as The Infection in a newspaper article) is an unknown virus that has caused the creation of the Infected. Although the infection seems to be more like a rabies-like pathogen. 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. Though apparently called Green Flu, the Infection does not behave like the flu, and graffiti reading "Not A Flu" can be found on the walls.

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. 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.

Contamination/ "Carriers"
In Left 4 Dead 2, it is revealed that people can be carriers. Carriers are immune to the virus, but carry it. It is written in graffitti 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 saferoom.

"Why are they seperating 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 seperate 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 wristwear. Graffitti stating that...

"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 callsign "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 has "room 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 weird, 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.

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 even cooking utensiles are fair game for the bug to live upon and will be transferred to anyone who would come into contact with it up afterwards. It is for this reason that even in the administration areas of Hospitals they are required to have hand sterilisation 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, 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 are "carriers" who were killed by CEDA, or more likely, the Military. There is also evidence in the form of graffitti in the "Hard Rain" campaign that known carriers were killed by uninfected civilians.



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 it's 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 immedietly, 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.

Extent
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 safezones indicated in the Northeast, Midwest, South and West, of which the West, Midwest and Northeast are crossed out, meaning they were besieged.

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.
 * 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. 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 bloodtype would be immune to the infection, that or most of the Immune Survivors were K.I.A.
 * As the cause of the Infection is unknown, some players have taken to calling it "Zombie Cancer" as a joke explanation, a reference to a Newgrounds cartoon called Awesome Center Redux.
 * 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.
 * 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.