AIDS (Acquired Immune Deficiency Syndrome) is a medical condition. People develop AIDS because HIV has damaged their natural defences against diseases.
HIV is a virus. Viruses infect the cells that make up the human body and replicate within those cells. A virus can also damage human cells, which is one of the things that can make a person ill.
HIV can be passed from one person to another. A person can become infected with HIV through contact with the body fluids of someone who already has HIV. HIV stands for the 'Human Immunodeficiency Virus' and the one who is diagnosed as infected with HIV is said to be 'HIV+' or 'HIV positive'.
The immune system is a group of cells and organs that protect your body by fighting disease. The human immune system usually finds and kills viruses fairly quickly. So if the body's immune system attacks and kills viruses, what's the problem?
Different viruses attack different parts of the body - some may attack the skin, others the lungs, and so on. The common cold is caused by a virus. What makes HIV so dangerous is that it attacks the immune system itself - the very thing that would normally get rid of a virus. It particularly attacks a special type of immune system cell known as a CD4 lymphocyte.
HIV evades the body's defences and mutates rapidly. This means that once HIV has taken hold, the immune system can never fully get rid of it.
There isn't any way to tell just by looking if someone's been infected by HIV. In fact a person infected with HIV may look and feel perfectly well for many years and may not know that they are infected. But as the person's immune system weakens they become increasingly vulnerable to illnesses, many of which they would previously have fought off easily.The only reliable way to tell whether someone has HIV is for them to take a blood test, which can detect infection.
A damaged immune system is not only more vulnerable to HIV, but also to the attacks of other infections. As time goes by, a person who has been infected with HIV is likely to become ill more and more often until, usually several years after infection, they become ill with one of a number of particularly severe illnesses.
It is at this point in the stages of HIV infection that they are said to have AIDS - when they first become seriously ill, or when the number of immune system cells left in their body drops below a particular point. Different countries have slightly different ways of defining the point at which a person is said to have AIDS rather than HIV.
AIDS is an extremely serious condition and at this stage the body has very little defence against any sort of infection.
Without drug treatment, HIV infection usually progresses to AIDS in an average of ten years. This average, though, is based on a person having a reasonable diet. Someone who is malnourished may progress to AIDS and death more rapidly.
Antiretroviral medication can prolong the time between HIV infection and the onset of AIDS. Modern combination therapy is highly effective and, theoretically, someone with HIV can live for a long time before it becomes AIDS. These medicines, however, are not widely available in many poor countries around the world and millions of people who cannot access medication die.