He became a commissioned officer in the U.S. Public Health Service (PHS) in 1939 and was assigned to the Division of Infectious Diseases, National Institute of Health. In 1943, he served in North Africa as a member of a medical team investigating endemic diseases, particularly malaria and typhus.
Dr. Davis returned to the Division of Infectious Diseases and in 1954, was appointed chief of the Laboratory of Infectious Diseases, National Microbiological Institute. In 1956, he was named associate director in charge of research for the Institute, now called the National Institute of Allergy and Infectious Diseases. In 1962, Dr. Davis became director of intramural research, and in 1964, NIAID Director.
In the 1960s, cellular immunology was offering scientists new ways to study various disorders. Dr. Davis established the first allergic disease centers at universities and medical centers around the United States to help translate basic research findings into new treatments. Nationwide research centers to study sexually transmitted diseases and influenza also were established. During this time, the Institute took over management of two international research programs: the U.S.-Japan Cooperative Medical Science Program and the International Centers for Medical Research and Training. Dr. Davis left NIAID in 1975 when he retired from PHS. He died April 11, 1990.