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 another story element (like the Green Flu) that has received no official or definitive explanation by Valve. Rather, there are random graffiti, in-game conversations, and posters throughout the game that suggest how carriers were handled by both CEDA and the military.

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 persons 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 advantagous. 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 detemined 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. Since everyone is dying from the infection eventually the virus will run out of hosts and the outbreak would be over, in theory. But as long as carriers continue to exist, the infection will exist and threaten the survival of immune individuals. Natural Selection will win eventually, but the only question is: will carriers or immune individuals remain?

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 inpenetrable vehicles i.e. helicoptors 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 grafitti that confirms the military were separating carriers from immune survivors but there has been no reasonable explanations as to what happened next. By the time the Parish Campaign had started, the military was carpet bombing entire cities. The airstrikes 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 explaning 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.