HIV

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 was contracted with HIV; CTU Los Angeles learned this through his medical history, and was given treatment for it. When it was learned Nina Myers had sex with Alvers, it is possible she may be contracted too.