Cancer researchers are increasingly turning to the microbiome to understand why some patients respond well to treatment while others face severe complications. Gut microbial communities shift during intensive therapies such as bone marrow transplantation, and those changes influence infection risk, immune recovery and long‑term survival. New advances in microbial sequencing and engineering redefine this community as a measurable clinical parameter that can be monitored, modeled, and even therapeutically reshaped to improve outcomes in oncology and other conditions.