Carriers

"THE ONLY GOOD CARRIER IS A DEAD CARRIER!"

- Graffiti in The Park

A Carrier is a person exposed to the Infection who appears asymptomatic due to a natural resistance to symptoms but lacks an immune response to degrade the disease. Without degradation, the person "carries" the disease in their body and can unintentionally transmit the disease to others rendering carriers a threat to public safety. The application of "Carriers" to the Left 4 Dead world is a story element that, until recently, was primarily shrouded in mystery. As presented in the most recent comic series "The Sacrifice," it appears that the carriers concept will play an important role in the future of the Left 4 Dead series. Aside evidence from the comic, there are random graffiti, in-game conversations, and posters throughout the game that suggest how carriers were handled by both CEDA and the military.

Confirmed Carriers
The following Left 4 Dead characters have been confirmed to be carriers of the Infection from the Sacrifice Comic:
 * Bill
 * Francis
 * Louis
 * Zoey
 * Military doctor

In the Sacrifice comic, the military doctor claims Bill and Zoey, as well as himself, are all carriers. Louis and Francis' status as carriers is never explicitly stated, however from the way Louis reacts in The Sacrifice comic (part 2), it is suggested that he received some (unseen) information (most likely from the guards) confirming himself and Francis to be carriers. It is unknown whether Coach, Nick, Rochelle, Ellis, are carriers or just immune (it seems that as far Valve is concerned though, "carriers" and "immunes" are the same thing, where 'immune' likely references innate immunity), however, considering that the helicopter pilot from the end of Dark Carnival became infected after rescuing the Survivors, it would pretty safe to say that they are carriers, if there is a distinction between the two in the Left 4 Dead universe. But then again, the pilot of the helicopter may have been infected before rescuing the Survivors (much like the helicopter pilot from "No Mercy") and cannot be definitively confirmed either way.

Carrier VS. Immunity
It is a highly misconceived notion that the terms carrier and immune are one and the same when in fact their meanings are very different. Actively immune individuals are people who are not resistant to the symptoms of a disease but have an immune response that fights off the disease and kills it. Put another way, immune individuals have had an immune system response to the disease and do not carry or can transmit the disease. Carriers have no immune response, usually due to a genetic factor or defect in the body that inhibits the disease from growing or causing symptoms.

However, the former is not possible in relation to the Infection. As stated in the comic, the virus mutates daily, which is far too little time for one's immune system to even begin forming antigens. Such an immune system would be superhuman, and the person who has it would likely never be ill of anything. Instead, the survivors' innate immunity is a genetic trait (as evidenced by the doctor claiming it 'runs on the father's side) and likely a recessive one, making it akin to color blindness in that it is bound to the X-chromosome and more common with men. This also explains the 3:1 gender ratio of the survivors and makes it clear that Zoey's father was, in fact, immune.

Continuing this thread of thought, the Green Flu infection is still present within each survivor's body, and though dormant, contaminating their saliva, breath, blood and whichever other channel the infection is transmitted through. It will not die off on its own, being viral. And since the survivors keep taking in loads of viral matter from the infected they encounter, it is nearly impossible for them to ever be cleansed of the infection. Conversely, it is fully possible that no matter how much the virus mutates, the survivors will still be immune to it, as long as it employs the same method of infection (probably infection of nerve cells and brain inflammation, as it is based on rabies).

As a final note, it is also possible for a survivor to be (innately) immune without being infected, though this would mean they've never come in contact with the infection, and finding out about their immunity would require extensive genetic research, more advanced than is possible with science as it is today.

CEDA/Containment
To learn of a carrier, there must have been proof that someone who was asymptomatic was infecting normal healthy people. Therefore, it's likely that CEDA inadvertently caused the infection of thousands by grouping all non-infected individuals together before realizing that there were carriers among them. The next step would have been to quarantine and test each asymptomatic survivor to see if they tested positive for the Green Flu. Such tests would likely have taken quite a while, and the general population would probably have little to no patience to wait. If CEDA was overrun by unruly citizens fleeing the infection, then containment would have failed and CEDA was likely pulled from running the show.

The Military
When containment turns into a calamity, desperate measures are necessary. This includes bringing in more powerful guns and deadly force into an already lose-lose scenario. From evidence presented in the games, the military was brought in with one thing on their mind, stop the spread of the infection by any means necessary. After consulting with the remaining members of CEDA, army medical scientists would have to first come up with a new scenario that helps identify carriers. Thus, a "save now, ask later" procedure would likely have been presumed as the the safest option. In such a parameter, any and all non-infected individuals were extracted from the contaminated cities using heavily armed and impenetrable vehicles i.e. helicopters and/or steel-plated/reinforced ATV's. Upon safe arrival at a central post, all survivors would be quarantined together and determined if they were immune or carriers. The next step is unknown. There is graffiti which supports that the military were separating carriers from immune survivors. At the same time, there is additional grafitti that claims the military were lining up carriers against a firing squad. Besides graffiti, there has been no concrete evidence as to what happened next. By the time the Parish Campaign had started, the military was carpet bombing entire cities. The air-strikes weren't very surgical (since most of the missiles miss the Survivors in the campaign) which suggests that the military had pulled out, surmising that any remaining evacuees had become infected or been killed. Otherwise, they would only be targeting infected instead of doing clean sweeps. As with other story elements in Left 4 Dead, there is an extremely large hole in explaining the military's involvement in containment and with carriers. As of the end of Left 4 Dead 2, the military were still in charge of containment and are therefore also in charge of carriers.