The first account of induced microbial drug resistance. Ehrlich encountered induced drug resistance in microbes while researching arsenical preparations as cures for sleeping sickness and other trypanosome-caused illnesses. His paper, delivered as a lecture on Feb. 13, 1907, “explained how the widely varying stains of trypanosomes, which at first reacted with great sensitivity to chemotherapeutic agents, gradually became drug resistant and how this property was passed on to their offspring for many generations” (Bäumler, Paul Ehrlich, p. 128). Includes an account of “Trypanrot”, by which Ehrlich succeeded in curing experimental trypanosomiasis. It was his work on this subject which led Ehrlich eventually to the production of Salvarsan.