The exact origin of HIV remains unknown, but most scientists agree that a form of the virus, known as SIVcpz (Simian Immunodeficiency Virus from chimps), was (and still is) present in chimpanzees living in parts of Africa.
One theory is that the virus spread to humans that were hunting the chimps, possibly because they came into close contact with the infected chimpanzee's blood. It is thought that for many years the human form of the HIV virus was limited to a remote part of Africa.
However, when new transport links opened up that part of Africa, the virus spread to other parts of Africa, before slowly spreading across the world.
How HIV spreads inside the body
The HIV virus breaks down the genetic code of cells used by our immune system, particularly the cells known as CD4 cells, and then uses the raw genetic material to make copies of itself. The body can make more CD4 cells, but eventually the HIV virus will reduce the numbers of CD4 cells to such an extent that the immune system will stop working.
How HIV spreads outside the body
The HIV virus can be spread through the exchange of bodily fluids, such as semen or blood. Therefore the condition can be spread through sexual intercourse, including oral and anal sex.
People who inject illegal drugs and share needles are also at risk from getting the HIV virus. The condition can also be spread from a mother to her unborn child. However, medicines can now be used to prevent this from happening.
HIV can be spread through blood transfusions. However, since 1985, it has been UK policy to screen all donated blood for HIV. Since the policy was introduced, no one in the UK has caught HIV from a blood transfusion.
Screening policies in the developing world may not be as rigorous as they are in countries such as the UK, so there is a possible risk of developing HIV if you receive a blood transfusion in certain parts of the world.
