Azidothymidine, known as AZT and later as zidovudine, was not made for AIDS. Chemist Jerome Horwitz synthesized it in Detroit in 1964, with a grant from the National Cancer Institute, as a possible cancer drug, and it proved unsuccessful for that purpose. Two decades later, after the virus behind AIDS was identified, as described in HIV Discovered, National Cancer Institute scientists Samuel Broder, Hiroaki Mitsuya and Robert Yarchoan began searching for compounds that could stop it in the laboratory, working with the drug company Burroughs Wellcome.

In February 1985 they found AZT active against the virus, and that year Mitsuya published the first report of its effect. AZT interferes with reverse transcriptase, an enzyme the virus needs to insert its genetic material into a host cell. Clinical trials sponsored jointly by the National Cancer Institute and Burroughs Wellcome followed. In September 1986 a trial of nearly 300 people with AIDS was stopped after 16 weeks because survival was much better on the drug: one…