[3] After completing his medical residency at the University of Chicago, he became a researcher at the National Cancer Institute, where he worked for 30 years, mainly as head of the Laboratory of Tumor Cell Biology.

[15] In 1982, Gallo received the Lasker Award: "For his pioneering studies that led to the discovery of the first human RNA tumor virus [the old name for retroviruses] and its association with certain leukemias and lymphomas.

[18] A French team at the Pasteur Institute in Paris, France, led by Luc Montagnier, had published a paper in Science in 1983, describing a retrovirus they called LAV (lymphadenopathy associated virus), isolated from a patient at risk for AIDS.

[16] In 1986, Gallo, Dharam Ablashi, and Syed Zaki Salahuddin discovered human herpesvirus 6 (HHV-6),[20] later found to cause Roseola infantum, an infantile disease.

[24] In 1995, Gallo with his colleagues Paolo Lusso and Fiorenza Cocchi published their discovery that chemokines, a class of naturally occurring compounds, are potent and specific HIV inhibitors.

Additionally, in 2011 Gallo and his team received $23.4 million from a consortium of funding sources to support the next phase of research into the Institute of Human Virology's (IHV) promising HIV/AIDS preventive vaccine candidate.

[30] Assignment of priority for the discovery of HIV has been controversial and was a subplot in the 1993 American television film docudrama (and earlier book about the early history of AIDS) And the Band Played On.

"[32] In 1989, the investigative journalist John Crewdson[33] suggested that Gallo's lab might have misappropriated a sample of HIV isolated at the Pasteur Institute by Montagnier's group.

[34] Investigations by the National Institutes of Health (NIH) and the HHS ultimately cleared Gallo's group of any wrongdoing[32][35] and demonstrated that they had numerous isolates of HIV of their own.

[36] On 12 December 1985 the Institut Pasteur filed suit to challenge a patent for an HIV test that had been granted on 28 May 1985 to the United States Department of Health and Human Services (HHS).

[32] In the November 29, 2002 issue of Science, Gallo and Montagnier published a series of articles, one of which was co-written by both scientists, in which they acknowledged the pivotal roles that each had played in the discovery of HIV,[38][39][40] as well as a historical review in the New England Journal of Medicine.

[44] In March 2011, Gallo founded the Global Virus Network in conjunction with William Hall of University College Dublin and Reinhard Kurth of the Robert Koch Institute.

The network's goals include increasing collaboration among virus scholars, expanding virologist training programs, and overcoming gaps in research, especially during the early stages of viral epidemics.