Letter from Colin M. MacLeod to Michael Heidelberger

During World War II Heidelberger served as a member of the Pneumonia Commission established by the Board for the Investigation and Control of Influenza and Other Infectious Diseases (later the Army Epidemiological Board) under the Surgeon General of the U.S. Army. The Commission, headed by Colin MacLeod, a microbiologist and co-discoverer of the genetic properties of DNA, organized a trial of a vaccine against pneumococcal pneumonia developed by Heidelberger. The trial, carried out among 20,000 trainees at an air base in Sioux Falls, South Dakota, proved that a vaccine made from a mixture of purified capsular polysaccharides from four different types of pneumococcus was effective against the disease.