HIV (Human Immunodeficiency Virus) is a virus that finds its way into white blood cells and replicates itself, thereby damaging the immune system. AIDS (Acquired Immune Deficiency Syndrome) is caused by HIV and is a variety of symptoms that weaken the immune system and allow for other infections to take hold. Not all HIV infections will lead to AIDS.

In Australia, approximately 85% of new HIV diagnoses are among gay and homosexually active men. HIV is mostly transmitted through infected blood or semen. The best course of prevention is through safe sex (use of condoms) and safe injecting (use of clean needles). For many years HIV rates in Australia have been falling, which has been linked to the gay community's activism around safe sex practices. However, a recent increase in the levels of unprotected sex among gay men has resulted in increased HIV infection rates in 2002 and 2003.

Due to improvements in HIV medicine, HIV positive people can live a lot longer than those diagnosed ten years ago, particularly if the virus is detected early. Late diagnosis can lead to an early transition to AIDS.

HIV blood tests are available from Sexual Health Services or GPs, and it's considered a good idea to test for a range of STIs (sexually transmissible infections) at the same time. It is recommended that men who have sex with men get tested for HIV and STIs at least annually, more often if necessary.

In the case of an incident involving HIV risk (ie. a condom breaking), there is a treatment available called PEP that may prevent the virus (see below).