William Cecil Campbell (born 28June 1930) is an Irish-American biologist and parasitologist known for his work in discovering a novel therapy against infections caused by roundworms, for which he was jointly awarded the 2015 Nobel Prize in Physiology or Medicine. He helped to discover a class of drugs called avermectins, whose derivatives have been shown to have "extraordinary efficacy" in treating River blindness and Lymphatic filariasis, among other parasitic diseases affecting animals and humans. Campbell worked at the Merck Institute for Therapeutic Research 1957–1990, and is currently a research fellow emeritus at Drew University.