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. 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.

Confirmed Carriers
The Sacrifice comic confirms the following characters as carriers of the Infection:
 * Bill
 * Francis
 * Louis
 * Zoey
 * Military doctor

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 enjoy natural immunity, the fact that the helicopter pilot at the end of Dark Carnival became infected after rescuing the Survivors provides circumstantial evidence that at least one of them is a carrier. Nevertheless this is not conclusive since it is possible that the pilot was infected before encountering the Left 4 Dead 2 Survivors.

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 he or she is both genetically resistant to the Infection and has 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.

CEDA/Containment
To be classified as a carrier, there must be proof that someone who was asymptomatic has infected normal healthy people. On this basis it is likely that CEDA inadvertently accelerated the epidemic's spread by grouping tens of thousands of hitherto uninfected individuals together at evacuation stations. Only later did the authorities realize that this process provided ideal cross-infection opportunities for the carriers caught up in the system. 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 have taken quite a while, the general population would probably have little to no patience to wait and the Infection would have been spreading at an accelerating geometric rate.

In the end, it is probable that CEDA's resources were overwhelmed by the scale of the disaster, the organisation itself was unravelling as the Infection caught on in the ranks of its staff and the social and logistical infrastructure around it slid 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).

The Military
When CEDA's measures prove to be ineffective and the epidemic turns into a seemingly unstoppable calamity the only open 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.

The next step is unconfirmed but graffiti evidence lends support to the notion that the military separated carriers from immunes. Additional graffiti and The Sacrifice comic strongly indicates that the military was summarily executing carriers by firing squad. Afterwards, the infected bodily remains of the carriers were sterilized via incineration. The Sacrifice comic reveals that the Survivors were sufficiently fearful of this fate as to prompt them to escape from military confinement and to thereafter eschew any contact with the military (refer Survivor dialog, The Passing campaign, L4D2).

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 commited 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 unravelling 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. An 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 cure or vaccine.