H - Human - because this virus can only infect human beings (A similar virus exists which is capable of infecting several types of monkeys; scientists call this virus SIV [Simian Immunodeficiency Virus]. Another similar disease exists among cats; this is caused by a virus named FIV [Feline Immunodeficiency Virus]).

I - Immuno-deficiency - because the effect of the virus is to create a deficiency, a failure to work properly, within the body's immune system.

V - Virus - because this organism is a virus, which means one of its characteristics is that it is incapable of reproducing by itself. It reproduces by taking over the machinery of the human cell.

HIV is the virus most researchers believe causes AIDS. However, some controversial scientists remain unconvinced that HIV is the cause of AIDS. Others believe that HIV can cause AIDS only in the presence of a "co-factor" -- some other virus or condition which has not yet been identified.

Scientists reported recently the existence of cases of people with severe immunodeficiency but with no evidence of HIV infection. Several researchers suspect the existence of a different virus. Some others speculate that these patients' immunodeficiency is due to other causes. But more research is needed to explain these cases.

However, since the vast majority of researchers believe that HIV is either the sole, or a primary, cause of AIDS, we often refer to HIV as "the AIDS virus."

A - Acquired - because it's a condition one must acquire or get infected with, not something transmitted through the genes

I - Immune - because it affects the body's immune system, the part of the body which usually works to fight off germs such as bacteria and viruses

D - Deficiency - because it makes the immune system deficient (makes it not work properly)

S - Syndrome - because someone with AIDS may experience a wide range of different diseases and opportunistic infections.

Opportunistic Infections Included in the CDC's Definition of AIDS

• Pneumocystis Carinii Pneumonia (PCP)
• Kaposi's Sarcoma (KS)
• HIV wasting syndrome
• Non-Hodgkin's lymphoma
• Cryptococcosis, extrapulmonary
• HIV encephalopathy (AIDS Dementia)
• Mycobacterium Avium Intracellulare (MAC or MAI)
• Candidiasis of the esophagus, trachea, bronchi, or lungs
• Cryptosporidiosis, chronic intestinal
• Cytomegalovirus disease (CMV)
• Tuberculosis (outside of the lungs)
• Herpes simplex virus infection
• Progressive Multifocal Leukoencephalopathy (PML)
• Primary lymphoma of the brain
• Toxoplasmosis of the brain
• Histoplasmosis
• Isoporiasis, chronic intestinal
• Coccidioidomycosis
• Salmonella septicemia
• Bacterial infections, recurrent, more than 13 years
• Lymphoid interstitial pneumonia/pulmonary lymphoid hyperplasia, more than 13 years.
• Pulmonary tuberculosis
• Recurrent bacterial pneumonia (two or more episodes in one year)
• Invasive cervical cancer

Note that more recent definitions finally include illnesses specific to women with HIV. Although this list has been revised several times since it was first developed, it does not include all of the illnesses experienced by people with HIV-related immune suppression.