AIDS (Acquired Immuno-Deficiency Syndrome) is a progressive and fatal condition caused by a breakdown of the body's immune system by a virus called HIV (Human Immuno-deficiency Virus). This condition leaves the subject vulnerable to a host of life-threatening opportunistic infections, neurological disorders, unusual malignancies, and a group of other manifestations and complications. It is probable that a person once infected will be infected for life. Strictly speaking, the term AIDS refers only to the late stages of the disease. This disease is referred to as a modern pandemic affecting people all over the world. Since reported first in 1981, AIDS has become a major worldwide concern.

We all live in an environment packed with microbes that are often harmful to health such as parasites, virus, bacteria, etc. All these are potentially capable of harming human body. But what shields the body from their bombardments is the immune system. The immune system in particular consists of certain categories of white cells in the blood, the lymphocytes, that endlessly patrol the body from their base, and the lymphoid organs.

As an organism invades the body, the T4 lymphocytes flash a signal to the T-lymphocytes and B-lymphocytes to counter the invasion. The T-lymphocytes directly attack the invader whereas the B-lymphocytes take help of antibodies that bind to the organism and destroy it. When these cells function normally, they help the body fight infections and diseases caused by viruses and bacteria. When the body is infected with HIV, the basic T4-lymphocytes, the command center of the immune system, get affected, thereby paralysing the body's defenses. AIDS begins when an individual is infected with HIV, continues with a phase in which a person has no signs or symptoms of the disease, and progresses to signs and symptoms that show a person’s immune system is no longer working properly. Therefore, AIDS is the advanced stage of HIV infection.. At this stage of the disease, many people develop opportunistic infections (OI) such as Pneumocystis carinii pneumonia, certain cancers such as Kaposi's sarcoma or develop other manifestations such as neurological involvement, weight loss syndrome etc. For people with AIDS, these OIs are often severe and sometimes fatal because the immune system is so damaged by HIV that the body can no longer fight off the bacteria, viruses, and other microbes that cause the OIs.