William George MacCallum (18 April 1874 – 3 February 1944) was a Canadian-American physician and pathologist. He was of Scottish descent and was born in Dunnville village in Canada, where his father was a physician. He was educated at the University of Toronto. He graduated with BA in 1894. Initially inclined towards Greeks as academic career, his father influenced him to enter medicine. He joined the second year of the first batch of medicine course in the Johns Hopkins Medical School, and became one of the first graduates of the institute in 1897. He was appointed assistant resident of pathology of the medical school in 1897, resident pathologist in 1901, soon after Associate Professor, and full Professor in 1908. Between 1909 and 1917 he held a twin position of Professor of Pathology at