‘AIDS’ as the definition of a terminal illness has in some ways fallen out of date since HIV therapy has enabled people to live in reasonably good health for many years, even if they have had a previous AIDS diagnosis. Some doctors these days prefer to talk about ‘symptomatic HIV infection’ as indicating a hopefully reversible effect of HIV.

However AIDS is still a useful definition in parts of the world where treatment is not so available, and as an indicator of the general health toll HIV is exacting on the population at a given time.

AIDS is also defined differently in different countries. In Europe it is still defined by the presence of one or more specific illnesses. In the USA, however, anyone with a CD4 count under 200 – about a fifth of the normal complement of CD4 cells – is deemed to have AIDS, even if they do not have current symptoms.

In reality, not all AIDS-defining disorders have the same prognosis, or outlook. For example, a gay man with a single lesion of Kaposi's sarcoma has AIDS, but has a much better outlook than a gay man with CMV disease or the brain infection PML. Age, race, gender and lifestyle factors can also mean that people who have the same defined HIV illnesses may have very different prospects.

Nevertheless, rigid definitions can be very useful in some circumstances. For example, in clinical and epidemiological studies, when large populations of people are being observed, it is essential to have well defined end points which mark the transition from one state of health to another. This is the only way that scientific principles can be observed and the studies can reach firm, reliable conclusions. It is also appropriate that these definitions change from time to time, as the epidemic evolves and we find out more.

However, definitions of HIV and AIDS as they have evolved have come to suggest that HIV infection is an inevitable, one-way process. In other words, they imply that everyone with HIV will initially be well, then they will get abnormal tests a little while before they get mild illness, and finally there will be a severe terminal illness.

This has been the pattern for many people, but others have had very different experiences. For example, people can get infections that would be diagnostic of AIDS and then become healthy again for a long while.