Human Immunodeficiency Virus (HIV) is the RNA-based virus that causes AIDS. It attacks the human immune system. Over time (and if effective treatment is not provided), HIV gradually destroys the body's defenses against certain diseases, leaving it vulnerable to what are called "opportunistic" infections and cancers that would not normally develop in healthy people.

Acquired Immune Deficiency Syndrome (AIDS) is a late stage of HIV infection. A syndrome is a group of symptoms and signs of disease that indicate a certain disorder when they occur together.

An HIV infected person receives an AIDS diagnosis when he or she has symptoms indicating severe immune system breakdown--either the person has fewer than 200 CD4 cells (a specific type of immune system cell which usually number 1,000 in most healthy people) or he/she has become ill or been diagnosed with one of 26 AIDS defining illnesses. Most AIDS-defining conditions are opportunistic cancers or infections.