Horstmann was born on July 2, 1911, in
Spokane, Washington and earned her undergraduate degree in 1936 from the
University of California, Berkeley. She received her medical training at the
University of California, San Francisco, earning her medical degree in 1940 and developed an interest in
infectious disease after hearing lectures delivered by
Karl Friedrich Meyer while at
San Francisco General Hospital, where she performed her
internship and
residency. She performed further training at
Vanderbilt University Hospital. Horstmann had initially been rejected from the residency program at Vanderbilt as the school's chief of medicine
Hugh Morgan only chose men to participate. Months later, she received a letter from Morgan asking whether "Dr. Horstmann" was still interested in the position. He obviously had forgotten that his original reason for exclusion of the applicant was because of gender. She replied with an acceptance of the position. When she showed up for work, Morgan "all but went into shock", but the year ended successfully. Hired by the Yale School of Medicine in 1942 as a Commonwealth Fellow in the Section of Preventive Medicine, Horstmann specialized in
internal medicine under Dr.
John R. Paul. Horstmann continued her work at Yale with a joint appointment in both the department of pediatrics and the department of epidemiology, which became part of a newly created Yale School of Public Health. ==Epidemiologist==