A Carrier is an asymptomatic host of the Rage Virus, who is immune to Infection's symptoms but can still transmit the disease to others.
Rare, certain people such as Alice and Andy Harris possess a genetic abnormality which acts as a natural immunity to the effects of the Rage Virus. Because of this immunity to the Rage Virus' symptoms, if and when one of these Carrier people is exposed to Infection and gets infected, while their blood and saliva will still carry the Virus and be able to transmit it to others, the Carrier his/herself will not exhibit any of the psychological or physiological symptoms of Rage seen in its non-immune hosts, except that the sclera of the Carrier's left eye will partially turn red and bloodshot. (28 Weeks Later)
There have only been two known Carriers - Alice Harris and her son Andy (28 Weeks Later) - although the black-ops' awareness of Carriers would imply that there are other Carriers besides Andy and Alice. (28 Days Later (comic series)) Alice Harris was killed by her infected husband Don and was incinerated during the Code Red firebombing, and Andy's current status and whereabouts after fleeing quarantined Britain for Europe are unknown. (28 Weeks Later)
Scarlet Levy believed that a Carrier's blood could be used to find a vaccine or possibly even a cure for Rage, but Alice was killed before any research could be conducted, and Andy's current fate and whereabouts are unknown. (28 Weeks Later)
- Though never specified or confirmed, some 28 Weeks Later fans have suggested that the genetic abnormality that gives Carriers their immunity to the symptoms of Infection may be the same one that causes the only two known Carriers' (Alice and Andy's) hetereochromic green-and-brown eye colour.