AIDS (acquired immunodeficiency syndrome) is a life-threatening disease that results from severe damage to part of the body's immune system. This system serves as a defense against infections and some cancers. AIDS is caused by infection with the human immunodeficiency virus (HIV). HIV gradually destroys certain white blood cells. The loss of these cells results in the body's inability to control microbial organisms that the normal immune system would resist easily. As a result, a person with HIV cannot fight infections. These infections are called opportunistic because they take advantage of damage to the immune system. For example, pneumocystis pneumonia is a type of lung infection that does not sicken people with normal immune systems, but that nearly all people with AIDS will get unless they take medicines to prevent it. Patients with HIV also frequently have certain cancers, such as Kaposi's...