AIDS stands for acquired immune deficiency syndrome.

AIDS is the result of damage to the immune system. A damaged immune system is unable to protect the body against certain specific 'opportunistic' infections and tumours. These are called opportunistic because they are caused by organisms normally controlled by the immune system, but which 'take the opportunity' to cause disease if the immune system has been damaged.

Unlike many other diseases, different people with AIDS may experience different clinical problems, depending on which specific opportunistic infections they develop. This is what a syndrome means - a collection of different signs and symptoms that are all part of the same underlying medical condition.