[2] He spent over two decades at the National Institutes of Health,[3] and has devoted much of his medical career to the diagnosis, management, prevention and treatment of children with cancer and AIDS.

[4][7] Following medical school Pizzo completed an internship and residency in pediatrics at Children's Hospital Boston.

[8] In 1976, after his fellowship ended, he was appointed to work at NIH as an investigator in pediatric oncology in the US Public Health Service.

[9] At the time of his appointment, NCI focused on cancer, but Pizzo realized the focus should also be on HIV, as the AIDS epidemic had begun to unfold, and he influenced the NIH leaders to take on the care of pediatric HIV cases, developing new approaches to treating and preventing AIDS in children.

[11] In 2013 Pizzo created the Stanford Distinguished Careers Institute (DCI), serving as its founding director, and was also named the David and Susan Heckerman Professor of Pediatrics and of Microbiology and Immunology.

[2][5][14] Pizzo is credited with leading the initiative in the late 1990s to secure federal funding for resident training in children's hospitals, which is roughly $300 million per year.

[17] From 2019 - 2022, Pizzo served as co-chair of the National Academy of Medicine Initiative on Climate and Human Health.