Health workers in the developing world may need to test adolescents routinely for HIV acquired "vertically" — through mother-to-child transmission, following findings published in the medical journal; Public Library of Science – Medicine. Nearly half of a group of three hundred patients between the ages of 10 and 18, admitted to hospital in Zimbabwe for any reason, tested positive for HIV. And the absence of herpes simplex infection in the majority of these — along with other factors — clearly indicates that sex was not the principal means of transmission. Rashida Ferrand discusses the findings of her London School of Hygiene and Tropical Medicine team working jointly with the Biomedical Research Institute in Harrare.