A Carrier (short for Asymptomatic 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. This renders Carriers a threat to public safety.
The application of the term carriers to the Left 4 Dead world is a story element that, until recently, was shrouded in mystery. However The Sacrifice comic proved its existence and it is likely that the Carriers concept will play an important future role in any Left 4 Dead sequels.
Aside from evidence in the comic, random graffiti, in-game conversations and safe room posters suggest how Carriers were handled by both CEDA and the military. Strong hints that the military summarily executed Carriers appear in Left 4 Dead 2.
The Sacrifice comic confirms the following characters as carriers of the Infection:
In the Sacrifice comic the military doctor claims that he, Bill and Zoey are Carriers. Louis and Francis' status as Carriers is never explicitly stated, however, from the way Louis reacts in part 2 of The Sacrifice comic, it is suggested that he received confirmation (most likely from the guards) that he and Francis are Carriers.
Although it is unknown whether Left 4 Dead 2 characters Coach, Nick, Rochelle, and Ellis are Carriers or just have a natural immunity, there are several factors that suggest that they are Carriers. After being picked up by the helicopter pilot of News Chopper 5 following the events of Dark Carnival, Nick is forced to shoot the pilot after he turns into a Common Infected, suggesting either that the Survivors themselves were Carriers, or he had been infected prior to rescuing them from another group he had saved. As they approach Veterans Memorial Bridge during the events of The Parish, they interrupt a transmission between Papa Gator and Rescue 7. Papa Gator asks them if they are infected, and the Survivors state that they are not infected, only for Papa Gator to reiterate if they had encountered the Infected. Should Coach be the one to grab the radio, he will state that they are covered in various body parts and organs on their trip to New Orleans. Regardless of whoever grabs the radio, Papa Gator will ask Rescue 7 if they are equipped for carriers.
In contrast to social and moral risks associated with being a carrier, a person who is an "immune" is one who enjoys innate immunity whereby they are both genetically resistant to the Infection and have an immune response that precludes that person from being a Carrier.
It seems that as far Valve is concerned the concepts of carrier and immune are synonymous and are treated with pragmatic equivalence in the storyline revealed thus far.
The reason for Valve intermixing 'Carrier' and 'Immune' is likely due to the fact that there is no true immunity to the ever-changing Infection. Anyone who does not see negative side-effects/ turns after encountering the Infected are most likely only immune due to being a Carrier.
Based on the Doctor saying the Infection has rapid changes, that makes it even more likely that it impossible to be immune, and everyone will either turn into a Common, Special, or Carrier Infected.
In the end, it is probable that CEDA's resources were overwhelmed by the scale of the disaster, the organization itself was unraveling as the Infection caught on in the ranks of its staff causing the social and logistical infrastructure around it to slide into chaos. Presumably therefore a top level decision was taken to suspend CEDA's operational control, declare martial law and to pass authority over to the military. In these circumstances, suspected Carriers were no longer routinely isolated and tested - but summarily executed and their corpses either cremated (refer The Sacrifice comic) or simply abandoned in evacuated zones (refer L4D2, The Parish chapter).
When CEDA's measures proved to be ineffective and the epidemic turned into a seemingly unstoppable calamity, the only option left is to suspend the affected region's civil administration. A declaration of martial law hands all operational responsibility and authority to the military whose personnel are indemnified for their actions and whose Joint Chiefs are accountable only to the Head of State.
From evidence presented in the games, the military was brought in and given a single objective: stop the spread of the Infection by any means necessary. After consulting with the remaining members of CEDA, army medical scientists would have first come up with a new operating procedure aimed at identifying individual Carriers. This "save now, ask later" procedure would have initially appeared to be the safest option. All uninfected individuals were evacuated from collection points under quarantine conditions using helicopters and armored personnel carriers equipped with sealed compartments and filtered ventilation systems. Upon arrival at a central post, these Survivors would be placed in lock-down quarantine together, tested and monitored to determine their status as either Immunes or Carriers. Afterwards, those seen as Infected or Carriers were killed and their bodily remains were sterilized via incineration (as seen in Millhaven).
The next step is unconfirmed but graffiti evidence lends support to the notion that the military separated the remaining Carriers from Immunes. Additional graffiti and The Sacrifice comic strongly indicates that the military was summarily executing Carriers by firing squad. The Sacrifice comic reveals that the Survivors were sufficiently fearful of this fate, after Millhaven's resident doctor informed them of this. Following their escape from the facility, the Left 4 Dead Survivors decided to go on their own and avoid all military, as shown in The Passing where they kindly deny the Left 4 Dead 2 Survivors offer to join them on their trip to New Orleans.
By the time of the The Parish, the military had evidently given up on its initial policy and was now abandoning major cities (e.g. New Orleans) and subjecting these centers to aerial bombing. These air-strikes appear to be more in the nature of nominal and symbolic public relations gestures for the news cameras since they are random in nature and can by no means be described as surgical. There is no evidence that the military is engaged in ground operations to combat the Infected with the aim of taking and holding hitherto overrun areas. On the contrary, the military appears to be committed to yielding territory and engaging in a largely ineffective scorched earth policy. Together with evidence provided in The Sacrifice comic, this suggests that the military does not have sufficient resources and, even worse, its own command and control infrastructure may be unraveling as its personnel succumb to the Infection and discipline within its ranks begins to crack.
As with other story elements in Left 4 Dead, there are more questions than answers concerning the military's operational policies regarding Carriers and Immunes. For example, at the end of Left 4 Dead 2 the military is still very much in control of containment resources, was able to lay on a special evacuation flight for the Survivors and would therefore appear to have a close interest in civilians who are either Carriers or Immunes. A hypothesis in this regard is that the military would wish to preserve the lives of a selected number of both so that comprehensive medical research can be conducted on them with the focused aim of developing a vaccine. Another may be that that Carriers, largely immune to infection, would be able to hold an actual frontline and allow non carrier soldiers to fight from a distance, allowing the military to actually fight the infection conventionally.
- A comment from Bill in Mercy Hospital in the elevator suggests that his group of Survivors, including himself, could turn at any moment. This indicates that he may be concerned that they are all Carriers who are just extremely resistant to the Infection, but may turn eventually.
- It is unlikely Bill knew anything about the Infection during this campaign.
- As heard in the beta, Bill seemed to be quite paranoid that any of the Survivors could turn into the Infected at any moment.
- Alternatively, Bill may just be concerned that the Survivors' immunity may not last forever, and that he does not want to live if he becomes an Infected.
- As told in the comic, the Carrier gene is passed down from the father, which is why Zoey had a breakdown in the comic when she learns that she killed her father at his request due to his belief he was Infected.
- Carriers are Special Infected. Following the same rules of every other Special Infected, the Carriers have a special name, a special ability, and transfer the Infection to others.
- The lack of Negative Side-effects/ Mutation is their special ability. This makes them a hidden "chess piece" in the Green Flu's wide-spread battle.
- It is unknown exactly how the virus spreads from a Carrier to an uninfected person. CEDA signs and Survivor graffiti has confirmed that it was originally believed to be an airborne virus, but isn't. A soldier also panics when Zoey knocks off his protective mask by saying, "Don't breathe on me..don't bleed on me..don't touch me", which indicates that perhaps the Infection can spread through either the air, fluids, or by direct contact.
- The military doctor in The Sacrifice comic contradicts the anti-airborne theory by telling the Survivors that "sometimes it's airborne, sometimes it's not." He follows-up by telling them that the Infection continues to mutate every day, basically saying it's unpredictable, making it very hard to make a vaccine for it.
- It is suggested that the Infection can spread through fluids, such as blood or bile. It is unclear if Boomer Bile can spread the Infection.
- It is also implied through the quarantine of Carriers and the "Wash your hands" signs that the Infection spreads upon contact. This is further supported by the Church Guy, who became Infected after he was bitten. Zoey's mother seems to have been Infected through a bite, and she turns soon after being bitten.
- When the Survivors in Left 4 Dead 2 tell the soldiers that they are immune (during The Bridge chapter), Papa Gator asks Rescue 7 if he is equipped for Carriers, which Rescue 7 acknowledges. This indicates that they are concerned that the Survivors are Carriers. However, seeing as they are covered in Infected blood, guts, spit, and bile, it is likely that the military would need an Infection-proof helicopter to transport them until they can be decontaminated.
- The graffiti "THE ONLY GOOD CARRIER IS A DEAD CARRIER" might be a reference to the 1997 sci-fi movie, Starship Troopers, which itself is a reference to racist slogans prominent before and during the civils rights movement.
- Several pieces of graffiti indicate that the military are just lining people against a wall and shooting them in a firing-squad type of manner.
- Several other Survivors have countered this by saying that they are Infected and are being shot to protect everyone. Despite the military's harsh nature, Survivor graffiti states they still seem to prefer the military over CEDA.
- In the Sacrifice Comic, the Military refers to the Survivors as "Tango Mikes". It is unclear if this is a term for Carriers or Survivors.