The early spread

Here we will review how, from its central African crucible, HIV managed to disseminate throughout Africa, at the same time as it did so across the Atlantic. But first we need to review two epidemiological terms. As explained in Chapter 1, ‘incidence’ is a measure of new cases of HIV that occur among previously uninfected subjects over a period of time. The same individuals have to be tested repeatedly: this is time-consuming, expensive and rarely used. ‘Prevalence’ is the proportion of individuals who have HIV at some point in time, a snapshot that indicates the current distribution. As the median interval between HIV infection and death in Africa is about ten years, measures of HIV prevalence reflect an accumulation of individuals infected from as little as a few weeks ago to more than ten years earlier. Over time, prevalence in a population increases if the number of new infections since the previous survey was greater than the number of individuals who died from HIV or other causes. Prevalence decreases when the reverse occurs, i.e. the number of deaths is higher than the number of new infections.