Acquired Immunodeficiency Syndrome (AIDS) is a disease of the immune system. It is caused by the Human Immunodeficiency Virus (HIV). It is a peculiar and deadly infection because the virus infects and destroys the immune system. This exposes the sufferer to many diseases, most commonly cancer or pneumonia, that cause the AIDS sufferer's death.

HIV is a virus that is found in body fluids, especially blood and semen. It is a microscopic ball of protein that contains the virus' genetic material. The virus has molecules on its surface that allows it to attach to lymphocytes in the immune system. Once this happens, it enters the cell and effectively hides from the immune system while it reproduces before spreading to infect other lymphocytes. At this stage, an infected person carries the virus but shows no outward signs of AIDS. A blood test will confirm if they have the virus by checking for antibodies in their blood. An HIV positive person can pass on the virus to others by having sexual intercourse without using a condom or in direct blood-to-blood contact, most commonly when drug users share infected needles. An HIV positive mother can pass the virus to their unborn child as it develops in the womb.

HIV does not cause any symptoms for up to several years as it spreads through the lymphocytes that control the immune system. Eventually the virus destroys these cells and their body is left unprotected against other diseases. This stage is called Acquired Immunodeficiency Syndrome (AIDS) and lung infections, like pneumonia, or a rare skin cancer called Kaposi's sarcoma develop. It is these diseases that ultimately cause the AIDS sufferer's death.

There is currently no cure for HIV/AIDS. World-wide there are an estimated 42 million people who are infected with the virus and in the United Kingdom the number is 50,000. Treatments, like the antiviral medicine AZT, can slow the progress of HIV infection to full-blown AIDS. Often several medicines are taken together and their long-term use can prove very expensive. HIV/AIDS is a problem in some developing nations and access to adequate medicines is a problem, although recent agreements with manufacturers are aimed at making these medicines more widely available.

HIV/AIDS is a disease that is not easy to catch. Infections are mainly passed on during unprotected sex, either between a man and a woman or a man and a man. Injecting drug users transmit the virus if they share infected hypodermic syringes and, although it was once a problem, blood transfusions are now screened to make sure they do not contain the virus. Transmission in saliva is virtually impossible. The virus can cross the placenta in HIV positive pregnant mothers but medicines are available that can reduce the chance of the baby being born HIV positive.

Using a condom and not injecting drugs prevent infection with the virus. HIV vaccines are being developed but, because the virus evades the immune system, it is difficult to make one that gives successful and reliable immunity. Some vaccines are in the early stages of testing but it will be at least 5-10 years before a vaccine is available for general use.