In the worst generalized HIV epidemics in East and Southern Africa, from one-quarter to three-quarters of women aged 15 years can expect to be living with HIV or to have died with AIDS by age 40 years. This disaster continues in the face of massive HIV prevention programmes based on current inexact knowledge of HIV transmission pathways and risks. To stop this disaster, both the public and public health experts need better information about the specific factors that allow HIV to propagate so extensively in countries with generalized epidemics. This knowledge could be acquired by tracing HIV infections to their source – especially tracing HIV infections in women of all ages, and tracing unexplained HIV infections in children with HIV-negative mothers.