A chemist tested 605 failed compounds before number 606 cured syphilis

Paul Ehrlich coined the term 'magic bullet' for a drug that would hunt down disease-causing microbes while leaving a patient's own cells unharmed. After screening hundreds of arsenic-based compounds, his 606th, arsphenamine, finally proved effective against syphilis in 1909 and was marketed as Salvarsan, the first modern chemotherapy drug. Ehrlich had already shared the 1908 Nobel Prize in Physiology or Medicine for his earlier work on immunity.

— Paul Ehrlich, Paul Ehrlich — Salvarsan (arsphenamine) introduced 1910