HIV is the viral agent that causes the condition known as AIDS. It might help to decipher the abbreviations:

HIV = Human Immunodeficiency Virus
AIDS = Acquired Immunodeficiency Syndrome

In common use, the terms HIV and AIDS are often used interchangeably.

The distinction is made clinically because infection with HIV typically doesn’t immediately cause AIDS, strictly speaking–it can take several years for the presentation of AIDS to become apparent. Because of this, it’s estimated that ~25% of those with HIV/AIDS in the U.S. are unaware that they are infected.

The conundrum is that upon infection with HIV, many of the initial symptoms can be vague: fatigue, low grade fevers, night sweats, weight loss, and intermittent diarrhea are common, but all of these symptoms have many, more frequently seen causes, so unless some risk factors are apparent in a patient’s history, it can be missed*. Hence, many institutions are now adopting the policy that separate written consent is not required for testing, and implementing the highly-recommended opt-out strategy, particularly for pregnant women.

HIV causes a deficiency in an immune cell that is identified by the presence of a marker protein known as CD4, and in 1993 the surveillance case definition came to include that a patient has AIDS if their CD4 count in a lab sample dropped below 200. There are various other tests now in use to confirm infection with the virus, but as far as I know, that was the first clear-cut definition.

* There are also numerous circumstances where the patient may not feel comfortable discussing their sexual history.