Epidemiology is the study of the way diseases spread through populations. It monitors the spread of diseases and tries to predict how epidemics will grow and who is likely to be affected by them.

Epidemiologists monitor patterns of illness, and sometimes identify new diseases as a result of this work. AIDS is a notable example. The possibility that a new illness characterised by immune suppression might be emerging was first suggested when the Centers for Disease Control (CDC, now known as the Centers for Disease Control and Prevention) in the United States noticed an unusual number of requests for drugs to treat Pneumocystis pneumonia (PCP, fomerly known as Pneumocystiscarinii pneumonia), which at that time was a very rare condition.

Epidemiology tells us:

• How an epidemic has developed.
• Where the epidemic is now.
• How the epidemic is likely to develop.

A proper understanding of the current nature of the HIV epidemic and its likely development over the medium and long–term is crucial if services – including both care provision and HIV prevention activities – are to be properly planned, accurately targeted and effectively delivered.