Before HIV can attack, it has to get in. HIV lives in bodily fluids like semen, vaginal secretions, blood, and breast milk. A person who carries HIV can pass it to another through any of these, usually via sexual intercourse, breastfeeding, or the sharing of drug paraphernalia. Rarely, a person will contract HIV through blood transfusions. And while it is highly unlikely for people to acquire HIV through saliva, it is possible to pass it through oral sex. Once the virus is transferred, it attaches to its new host body's sex, or T-cells, which are integral parts of the immune system. Inside the T-cell, HIV literally changes to become part of the body's DNA, or genetic code. At this point, the body will be forced to produce the virus.