He was born in Waterbury, Connecticut.
Gallo's choice of profession was likely influenced by the early death of his sister of leukemia. He made bachelor in biology in 1959 and graduated from medical school in 1963. He studied the biosynthesis of hemoglobin, until he moved to the NIH in 1965, where he studied leukemia.
Gallo discovered the first retrovirus in humans in 1981 (human T-cell leukemia virus, HTLV), and in 1984, he published the discovery of HIV, which was dicovered by the French scientist Luc Montaigner.
While some in the scientific community praise his genius [1], a minority of scientists argue that the HIV discovery is a fraud (see Duesberg hypothesis).