Brendan J. Kelly, M.D., M.S., is an infectious diseases physician and epidemiologist at the University of Pennsylvania. He studies the microbial ecosystems of the human respiratory and gastrointestinal tracts, with a focus on properties that protect against bacterial pathogen colonization and infection, how microbial ecosystems are altered by antibiotics, and how they can be engineered to prevent and treat antibiotic-resistant infections. He has performed large, clinical-translational studies to better understand two infections that occur commonly during critical illness: bacterial pneumonia and C. difficile colitis. He has also investigated the use of fecal microbiota transplant (FMT) to aid recovery of the intestinal microbiota after C. difficile treatment and to prevent multidrug-resistant organism (MDRO) infection. During the COVID-19 pandemic, he has led a study of SARS-CoV-2 variant succession across the Philadelphia metropolitan region. His work has been supported by the National Institute of Allergy and Infectious Diseases (NIAID) and the Centers for Disease Control and Prevention (CDC).