American pediatrician and microbiologist whose groundbreaking work on influenzal meningitis significantly reduced infant death rates and advanced the field of microbiological genetics.

Alexander received her bachelor's degree in 1923 from Goucher College, in Towson, Maryland. Her undergraduate studies in bacteriology and physiology led to her first two jobs, as a public health bacteriologist, first for the national public health service and then for its state counterpart in Maryland. With her earnings from these jobs, she started medical school, receiving an M.D. from the Johns Hopkins School of Medicine. During her internship in pediatrics at Baltimore's Harriet Lane Home she developed what would become an enduring professional interest in influenzal meningitis, then a fatal disease. A successful residency at New York City's Babies Hospital, a facility at Columbia-Presbyterian Medical Center, led to her appointment as instructor in pediatrics. She remained associated with Columbia in the role of teacher, researcher, and practicing physician for the rest of her career. Under her control, the microbiology laboratory at Babies Hospital set a profession-wide standard.

Alexander's own research centred on influenzal meningitis. Building on a successful antipneumonia serum prepared in rabbits at New York City's Rockefeller Institute, Alexander in 1939 reported the first complete cure of infants suffering from influenzal meningitis. Over the next few years, Alexander's experiments with sulfa drugs and with various antibiotics resulted in a significant reduction in the infant death rate from influenzal meningitis. Her realization that some influenza bacilli cultures were resistant to antiobiotics as a result of genetic mutation directed her into the nascent field of microbiological genetics. In 1950, again building on work at Rockefeller Institute, Alexander and her colleague Grace Leidy reported their success in using DNA to alter the hereditary characteristics of Hemophilus influenzae, the cause of influenzal meningitis. Alexander in 1964 became the first woman president of the American Pediatric Society, and even after her retirement she continued to serve as a special lecturer in pediatrics and as a consultant to the Columbia-Presbyterian Hospital.