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 Columbia University and the NewYork–Presbyterian Hospital. From 1917 to 1943 he held the Chair of Pathogy at Johns Hopkins University.
MacCallum discovered the existence of two forms (now known to be male gamete or microgametocyte and female gamete or macrogametocyte) of malarial parasite in birds in 1897. In 1899 he and T.W. Hastings discovered a new species of pathogenic Gram-positive bacteria called Micrococcus zymogenes. He was the first to describe the structural and functional relationship between lymphatic system and connective tissue. In 1905 he discovered that thyroid and parathyroid glands had completely different functions. He found that muscle seizure (tetany) was due to abrogation of parathyroid glands, and that injection of calcium salt could restore the condition. This directly laid the understanding of the role of calcium in muscle contraction. In 1909 he discovered that a disease gastric tetany was not due to parathyroid functions, but because of blockage of the stomach-intestine connection called pylorus. He wrote a definitive textbook A Textbook of Pathology which ran several editions and is still in print.
A histochemical staining technique for Gram-negative bacteria called McCullum-Goodpasture stain is jointly named after him and the co-discoverer Ernest William Goodpasture. A condition in rheumatic heart disease MacCallum plaque is named after him.
William MacCallum was born in Dunnville village to a physician father George A. MacCallum and mother Florence Eakins. His grandfather George MacCallum had emigrated from Scotland to Canada in his youth. He was the second of four children, with an elder sister, and a younger brother and a sister. (His brother John Bruce MacCallum would also become a physician but died at the age of thirty due to tuberculosis). He entered Dunnville public school and high school for his early education. He spent much of his free time accompanying his father, who was a general practitioner, in visiting resident patients. At 15 years of age he entered the University of Toronto, where his main interest was in Greeks, among his subjects such as zoology, chemistry, physics and geology. He graduated in 1894. His father’s persuasion that he should take up medicine was his career-making event as he wanted to continue with Greeks for himself. By then he learned that the Johns Hopkins University had started Medical School in 1893 at Baltimore with its first batch of medical students. He applied for the medical course and was allowed to join the first batch, who were already in their second year, as it was decided that his education at Toronto was considered equivalent to the first year medical course. He was therefore member of the first class at Johns Hopkins who earned MD degree in 1897.
MacCallum served a one-year internship at Johns Hopkins Hospital during 1897-1898, and was appointed assistant resident pathologist under William Henry Welch at the Johns Hopkins University. In 1900 he worked with Felix Jacob Marchand at the University of Leipzig in Germany. He returned to Baltimore in 1901 to become resident patholosgist and Associate Professor of Pathology. He was promoted to full Professor in 1908. Between 1909 and 1917 he was invited to Columbia University and NewYork–Presbyterian Hospital, where he simultaneously worked as Professor and pathologist respectively. In 1917 Welch resigned from the Chair of Pathology at Johns Hopkins University to assume the new post of Director at the School of Hygiene and Public Health. MacCallum was selected to take the vacant Chair, which he occupied until his retirement in 1943. He was designated Baxler Professor of Pathology in the university and pathologist to the Johns Hopkins Hospital.