Human immunodeficiency virus (or simply known as HIV) is a treatable, but incurable sexually transmitted disease. People can usually contract the virus through unprotected sexual contact with one who is already exposed.
At some point before Day 3, Marcus Alvers contracted HIV and sought treatment for it; CTU Los Angeles learned this through his medical history. When it was learned Nina Myers had sex with Alvers, this information was used to convince her to divulge information about him.