Carriers

"THE ONLY GOOD CARRIER IS A DEAD CARRIER!"

- Graffiti in The Park

A Carrier is a person infected by the Infection who appears asymptomatic due to a natural resistance to symptoms but lack 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 term carriers 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. For example, H5N1 (commonly known as bird flu) is a strain of influenza that currently does not react with humans because the virus only has interaction capabilities with the cells of birds. A human exposed to the avian strain would have no immune response because human genetic factors are different than avians but the same person could potentially transmit the disease if the person comes into contact with birds.

CEDA
At first, CEDA attempted to quarantine the Carriers from others in an attempt to minimize the disease transmission. CEDA took in any Survivors of the apocalypse - immune, non-immunes and even (judging by graffiti) carriers. CEDA quarantines any Carriers they locate and attempt to keep them away from the rest of the Survivors. The reason for this is because while they may be immune and/or extremely resistant to the Infection, they can still pass it on to other Survivors, immune or otherwise. They possibly examine the Carriers further to analyse information regarding the Infection; therefore, this may explain why they merely quarantine the Carriers rather than deny access to Survivor camps to them.

The Military
When it was determined that CEDA weren't doing their job to the best of their capabilities, the military took over and took in all Survivors. Like CEDA, they would separate Carriers from other Survivors. Unlike CEDA, however, instead of examining Carriers for information regarding the Green Flu, they would simply shoot and kill Carriers to prevent the Infection from spreading. The military then also bombed entire towns and cities with fighter jets to slow down the spread of infection. However, it is possible that Survivors are caught up in the bombing runs, like the new Survivors were in New Orleans. Although this may have helped minimize the rate and spread of Infection, this may have also minimized the number of Survivors who hadn't yet made it to military encampments.