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 Sacrafice Comic:
 * Bill
 * Francis
 * Louis
 * Zoey

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, when they are not synonymous in real life. See below section for more details on this), 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 rather than immunes, if there is a distinction between the two in the Left 4 Dead universe.

Carrier VS. Immunity
It is a highly misconceived notion that the terms carrier and immune are one in the same when in fact their meanings are very different. Completely immune individuals are people who are not resistant to the symptoms of a disease but have an unnaturally fast and strong immune response that fights off the disease quickly. 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.

It's difficult to determine which is more advantageous. Just because someone is immune to a virus doesn't mean they're safe forever. Since an immune response depends on the body's detection of the virus (antigen detection), a previously immune person could become symptomatic in the likely case that the virus changes or mutates. In contrast, it's hard to say if a carrier would ever become symptomatic to a virus that has undergone changes or mutations. One certainly could but as said before, a carrier's lack of symptoms is typically determined by a discrepancy (usually genetic) in the human cells that prevent a virus from attaching. It's highly unlikely that a virus would be able to change accordingly. One must also consider the long term benefits of either being immune or a carrier. In theory, the outbreak will continue until it is contained or there are no more viable hosts. As long as carriers continue to exist, the infection exists and threatens the survival of immune and non-immune individuals. If society can't contain the outbreak then it can be assumed that natural selection will eventually. Either way, it remains to be seen if immune individuals or carriers will prevail over the other.

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 taken hours and when you're having to test hundreds of thousands of people, all while the apocalypse is occurring, 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.