William Henry Welch was a senior pathologist at Johns Hopkins University and its hospital. He researched numerous diseases, including pneumonia and diphtheria, but is most renowned for his discovery of the Bacillus welchii, a bacterium that causes gangrene. Throughout his career, Welch advocated asepsis and other general reforms in American hospitals to control disease and advance medical care.

Welch was born in Norfolk, Connecticut in 1850. He attended Yale and graduated in 1870. He then studied to be a surgeon at Columbia University, earning his M.D. in 1875. Welch then pursued advanced studies in Europe. He studied at several universities, but was perhaps most influenced by his time in Berlin. He returned to the United States in 1878 and was a professor and physician at Bellevue Hospital and Medical College in New York.

Welch conducted most of his career research as a professor and pathologist-in-chief at Johns Hopkins University and hospital. He accepted a position at the emerging hospital and medical school in 1884. His commitment to hospital reform and public health led to his discovery of the cause of gas gangrene. Later, Welch was named the director of the School of Hygiene and Public Health.

Welch's commitment to public health, as well as clinical medicine, garnered several awards, including the U.S. Army Distinguished Service Medal and Citation. Because gangrene was not only a serious surgical risk, but also an endemic problem with battle wounds, Welch's identification of Bacillus welchii was of military and medical interest.

In addition to his academic appointments, Welch held several offices in professional organizations. He founded the Journal of Experimental Medicine in 1896. Welch served on the Maryland State Board of Health for 31 years. He was president of the American Medical Association in 1910.

Welch died in 1934 while still serving on several medical boards.