Stewart joined the National Institutes of Health (NIH) from 1935-1944 while completing her PhD at the University of Chicago.[1] During her time there, she took part in developing a vaccine for gangrene, which helped many soldiers during the second world war.[2] She later left her position in the NHI in order to pursue her goals and research field, which was to prove that there was a connection between cancers and viruses.[3] This proved to be difficult because, at the time, not many scientist took this field seriously.[3] She had also recreated Ludwik Gross’ experiment with mice, which showed that the Leukemia found in certain mice was indeed caused by viruses; they released a paper together discussing their findings.[3] She went on to teach microbiology at Georgetown University's School of Medicine, and once women were allowed to enroll, she became their first female graduate at the age of 39.[1][5] Stewart returned to the NIH in 1951, joining the National Cancer Institute (NCI), and eventually becoming medical director.[1]