Estimated HIV incidence
'HIV incidence' is the number of new HIV infections in the population during a certain time period. People who were already infected before that time period are not included in the total - not even if they are still alive and HIV+.
Unfortunately, directly measuring HIV incidence is a complex and expensive process, so incidence data for many resource-poor areas, and some rich ones, is difficult to find.
However, one example is that (according to USAID) in the Masaka region of Uganda, HIV incidence fell from 7.6 per thousand per year in 1990 to 3.2 per thousand per year by 1998.
National estimates of HIV incidence are usually produced by computer models and are based on estimates of HIV prevalence. Such models apply a set of assumptions such as the survival time of those infected with HIV and the mother-to-child transmission rate. Trends in HIV prevalence among young people (who are likely to have been recently infected) can give a rough idea of the incidence rate in this group.
