AZT and other medications that work as reverse transcriptase inhibitors interfere with DNA replication in HIV/AIDS cells, which prevents reproduction of these cells. Though it does not cure this illness, it does interfere with the rapid mutation cycle which, in turn, decreases the amount of illnesses suffered by the patient. This makes it possible for the patient to survive with this virus/disease longer and remain in a relatively healthy state. Though, as already mentioned, the sufferer is still infected and does need to take the proper precautions to avoid spreading the infection.
