William Cecil Campbell FRS[1] (born 28 June 1930) is an Irish-American microbiologist 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.

[2] 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.

He then attended the University of Wisconsin–Madison on a Fulbright Scholarship, earning his PhD degree in 1957 for work on the liver fluke, a parasite affecting sheep.

Campbell led a team at Merck in studying Ōmura's cultures and examining their effectiveness in treating parasites in domestic and farm animals.

[14] As of 2001[update] an estimated 25 million people were being treated each year, in a total of 33 countries in sub-Saharan Africa, Latin America, and the Middle East.

[13] He has written about the history of parasitology in Antarctic exploration, including the work of surgeon Edward L. Atkinson in Scott's ill-fated Terra Nova Expedition.

[21] In 2015, he and Satoshi Ōmura shared half of the 2015 Nobel Prize in Physiology or Medicine for their research on therapies against infections caused by roundworm parasites, using derivatives of avermectin.