American pathologist who played a major role in the introduction of modern medical practice and education to the United States while directing the rise of Johns Hopkins University, Baltimore, to a leading position among the nation's medical centres.

Undertaking graduate medical study in Germany (1876–78), Welch was working in the laboratory of the pathologist Julius Cohnheim at the University of Breslau when he witnessed Robert Koch's historic demonstration of the infectivity of Bacillus anthracis. Returning to the United States, Welch became professor of pathology and anatomy at Bellevue Hospital Medical College, New York City (1879), and five years later he developed the first true university department of pathology in the United States at the newly created Johns Hopkins University. There he was instrumental in recruiting for the faculty the famed physician William Osler (Osler, Sir William, Baronet) and the surgeon William Halsted (Halsted, William Stewart). As the university medical school's first dean (1893–98), Welch virtually single-handedly constructed a curriculum that revolutionized American medicine by demanding of its students a rigorous study of physical sciences and an active involvement in clinical duties and laboratory work. He numbered among his students the yellow-fever investigators Walter Reed and James Carroll and the bacteriologist Simon Flexner.

As an original investigator, Welch is best known for his demonstration (with Flexner; 1891–92) of the pathological effects produced by diphtheria toxin and for his discovery (1892) of Micrococcus albus and its relation to wound fever and of Clostridium welchii (Welch's bacillus), the causative agent of gas gangrene.