Harold Sheely Diehl (August 4, 1891 – June 27, 1973) was an American physician, anti-smoking activist, public health educator and writer.

[1] He became Professor of Preventive Medicine and Public Health and was appointed dean of Medical Sciences.

[1] In 1938, Diehl and colleagues published results from a trial on the efficacy of vaccines for the common cold.

"[10] In 1970, the book was positively reviewed in the British Medical Journal, which concluded, "cause and effect are established beyond reasonable doubt by the overwhelming weight of evidence linking cigarette smoking with serious disease... [t]his book should effectively reinforce the movement which must one day turn public opinion against the cigarette, or stimulate the development of a safe one.

"[11] Diehl criticized dietary fads such as the alkaline diet, food combining, raw foodism, strict vegetarianism and water fasting.