For generations, cancer was understood as a disease of rogue cells — a story told entirely in the language of human biology. Now, researchers are discovering that tumors are not sterile fortresses but living ecosystems, inhabited by bacteria, fungi, and viruses that quietly govern how cancer grows, how the immune system responds, and whether treatment succeeds. This ecological reckoning is reshaping oncology's most foundational assumptions, opening a new chapter in which the microbial world inside a tumor is not a footnote but a central character in the story of disease and healing.