In reality, not all `AIDS defining' disorders have the same prognosis, or outlook. For example, a man with a single lesion of Kaposi's sarcoma has AIDS, but has a better outlook than a man with Pneumocystis pneumonia (PCP), another common AIDS-defining disorder. Age, race, gender and lifestyle factors can also mean that people who have the same rigidly 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.

Definitions of HIV and AIDS as they have evolved have come to suggest that HIV infection is an inevitable, one-way process. The implication is that people with HIV will initially be well, will then get abnormal tests a little while before getting mild illness, and that 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. Furthermore, many people with HIV infection have never experienced any symptomatic diseases even after many years.