The Aids epidemic may have been triggered by the mass vaccination campaign which eradicated smallpox. The World Health Organization, which masterminded the 13-year campaign, is studying new scientific evidence suggesting that immunization with the smallpox vaccine Vaccinia awakened the unsuspected, dormant human immuno defence virus infection (HIV). Doctors who accept the connection between the anti-smallpox campaign and the Aids epidemic now see answers to questions which had baffled them. How, for instance, the Aids organism, previously regarded by scientists as 'weak, slow and vulnerable,' began to behave like a type capable of creating a plague. The smallpox vaccine theory would account for the position of each of the seven Central African states which top the league table of most-affected countries; why Brazil became the most afflicted Latin American country; and how Haiti became the route for the spread of Aids to the US. The greatest spread of HIV infection coincides with the most intense immunization programmes. Although detailed figures of Aids cases in Africa are difficult to collect, the more than two million carriers, and 50,000 deaths...are concentrated in the Countries where the smallpox immunization programme was most intensive. Brazil, the only South American country covered in the eradication campaign, has the highest incidence of Aids in that region.