HIV infection and AIDS

HIV (Human Immunodeficiency Virus)

HIV is a virus which acts by depleting the body’s normal immunological defence mechanism.  Over time, people with HIV become increasingly likely to develop infections, or some forms of cancer, which the body would normally be able to fight.

HIV is passed from one person to another through infected blood, semen, vaginal fluids and breast milk. It is passed on in three main ways:

  • through unprotected anal and vaginal sex
  • through sharing contaminated injecting equipment (needles and syringes)
  • from an HIV-positive mother to her unborn baby or through breast feeding.

A person is generally diagnosed as having HIV on the basis of a positive HIV antibody test.

AIDS (Acquired Immune Deficiency Syndrome)

AIDS is the late stage of the spectrum of disease caused by HIV infection.  

For surveillance purposes in New Zealand, and most other countries, a person with HIV infection is said to have developed AIDS when one or more of a list of 25 AIDS defining illnesses are present that are uncommon in people with normal immunity. 

In the US however, a person is defined as having AIDS when their CD4+ lymphocytes – a groups of circulating cells that are depleted by HIV – are fewer than 200 cells per cubic millimeter of blood.  Because of the different way AIDS is defined care needs to be taken in comparing the data on AIDS in the US (which tends to have higher numbers) with that of other countries.

University of Otago Dunedin School of Medicine University of Otago AIDS Epidemiology Group