What is AIDS?

Before we can begin to look for a cause, we must first work out exactly what type of illness we are talking about.

In early 1981, doctors in New York and California began to report some bizarre new disease outbreaks. In both places, previously healthy young men were showing up with rare illnesses including Kaposi's sarcoma (a kind of tumour) and PCP (a type of pneumonia), which until then had been virtually unheard of among such people. Within months, dozens of similar cases had been reported in 23 American states and in the UK, representing the start of a massive and unprecedented epidemic.⁵

Doctors soon discovered a distinctive feature of these cases. More than anything else, the men were lacking a specific type of white blood cell, which is essential to a healthy immune system. Normally, people have between 600 and 1,500 "CD4+ cells" (also called T helper cells) in each cubic millimetre of their blood. But the men with the strange new disease typically had very much lower levels. This immune deficiency explained why they were so vulnerable to disease.

The cases were clearly related in time and by population group (initially gay men and injecting drug users). No cause of immune deficiency could be found, but it was clearly not inherited. Scientists therefore grouped together all of these strange new cases under the heading "Acquired Immune Deficiency Syndrome" – or AIDS for short.

In 1982, no-one claimed to know the cause of AIDS, so the first definition was based on the diagnosis of one of 13 rare diseases known to be linked to immune deficiency (including Kaposi's sarcoma and PCP) "occurring in a person with no known cause for diminished resistance to that disease".⁶ Over the years, the US definition has been refined as hundreds of thousands of similar cases have been documented, sometimes involving other diseases, but always associated with the same distinctive immune deficiency.⁷ Other definitions have also been developed to suit different situations elsewhere in the world.⁸

The latest US AIDS definition was created in 1993. Under this definition, someone has AIDS if they have one of 26 specific diseases (28 in children) but no known cause of immune deficiency other than HIV (with some diseases, a positive HIV test is required); or if they have a CD4+ cell count below 200 cells per cubic millimetre of blood, or less than 14% of all lymphocytes, plus a positive HIV test.⁹

Europe and Canada have similar AIDS definitions to the US, but do not include low CD4+ cell counts.