HIV stands for Human Immunodeficiency Virus. It is a virus which attacks certain cells, called 'CD4' cells, in the person's immune system. The immune system fights off infections and cancer.
If someone has HIV, the virus attacks their CD4 cells. The immune system tries to fight back by making new CD4 cells.
However over a period of about 8-10 years, the HIV gradually 'wins' and the number of CD4 cells falls to low levels.
Below a certain level, the person becomes more likely to fall ill with infection or cancer. When the person becomes sick the doctors say he / she has reached the stage of AIDS.
If someone knows they have HIV they can get regular blood tests to monitor the level of the CD4 cells. This means they can start anti-HIV treatment at the right time to prevent illness and stay well. If treatment is taken correctly, then life expectancy is near normal.
They can also be extra careful not to pass HIV on to anyone else. Partners, ex-partners and sometimes children, can be offered an HIV test.
HIV can only be passed on if the virus gets from one person's body fluids into the bloodstream of another person.
This only happens in certain ways:
by vaginal fluid, menstrual period blood, semen, or pre-cum during unprotected vaginal or anal sex (i.e. without a condom or Femidom),
by blood through sharing drug injecting equipment,
by blood transfusion (all blood products have been screened in the UK since 1985)
from mother to baby, during pregnancy, at birth, or through breast-feeding.
The risk of picking up HIV from oral sex (either giving or receiving) seems to be very small, but there are a few cases where it does seem to have been passed in this way.