Carriers

"THE ONLY GOOD CARRIER IS A DEAD CARRIER!"

- Graffiti in The Park

Carriers are Survivors who bear the Infection, but are either immune or extremely resistant to its effects. However, they may bear a threat to non-immune Survivors due to the fact that they can spread the Infection and, therefore, infect the non-immunes and cause them to turn. CEDA attempted to counter this by quarantining the Carriers and separating them from the other Survivors in an attempt to minimize the amount of Infected Survivors. The military, however, seem to shoot and kill Carriers in an attempt to keep the Infected population from growing still further.

CEDA
CEDA take 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.