George Keble Hirst, M.D. (March 2, 1909 – January 22, 1994) was an American virologist and science administrator who was among the first to study the molecular biology and genetics of animal viruses, especially influenza virus. He directed the Public Health Research Institute in New York City (1956–1981), and was also the founding editor-in-chief of Virology, the first English-language journal to focus on viruses. He is particularly known for inventing the hemagglutination assay, a simple method for quantifying viruses, and adapting it into the hemagglutination inhibition assay, which measures virus-specific antibodies in serum. He was the first to discover that viruses can contain enzymes, and the first to propose that virus genomes can consist of discontinuous segments. The New York Times described him as "a pioneer in molecular virology."
Hirst was born in Eau Claire, Wisconsin, USA, but his family soon moved to Lewistown, Montana. He studied at Hobart College in Geneva, New York, and later at Yale University, New Haven, Connecticut, from which he gained his first degree and medical degree (1933). He worked at the Rockefeller Institute for Medical Research in New York City in 1936–1940, under the supervision of Homer Swift and Rebecca Lancefield, and then moved to the Rockefeller Foundation's International Health Division laboratories in 1940. There he combined research in collaboration with Frank Horsfall, Edwin D. Kilbourne and others with his army service, as a member of the Armed Forces Epidemiological Board's Commission on Influenza.