His research focuses on the human microbiome and microbial ecosystem—for which he was a pioneer in the use of modern molecular methods, as well as on pathogen discovery and the genomics of host response.

[1] Relman was born in Boston, raised in Lexington, MA, and then moved to Philadelphia where he attended Germantown Friends School.

[4][5] His current research seeks to elucidate the nature and basis of diversity, assembly,[6] stability and resilience[7][8] in the human microbial ecosystem, and their relationships with health and disease.

In 2019-20, Relman chaired a committee of the U.S. National Academies of Sciences, Engineering, and Medicine that examined unexplained clinical signs, symptoms, and findings amongst personnel and their families working for the U.S. Department of State in Havana and in China.

[14][15][16] In 2020, Relman called for an objective, dispassionate, and transparent investigation into the origins of COVID-19 and SARS-CoV-2,[17] noting that despite many strongly-worded opinions and assertions about the likelihood of various hypotheses, "the 'origin story' is missing many key details, including a plausible and suitably detailed recent evolutionary history of the virus, the identity and provenance of its most recent ancestors, and surprisingly, the place, time, and mechanism of transmission of the first human infection", and that natural and laboratory-associated "spillover" scenarios both remained plausible.

[19] Relman was a long-time volunteer for the Rock Medicine program organized by the Haight Ashbury Free Clinics, and served as its Chief Medical Officer for about a decade 2006.