Cancer is your own cells, mutated into refusing to stop dividing

Mukherjee's foundational definition is that cancer isn't an invading foreign organism like a virus or bacterium — it's a person's own cells that have accumulated genetic mutations disrupting the normal controls on cell division, growth, and death, causing them to multiply uncontrollably and, in malignant cases, invade surrounding tissue and spread to distant organs.

He explains that healthy cells operate under tight genetic regulation: genes that promote growth (oncogenes, when mutated) and genes that suppress it (tumor suppressor genes, when disabled) normally balance each other, along with built-in cellular mechanisms for self-destruction when something goes wrong. Cancer typically requires multiple mutations accumulating in the same cell lineage over time, disabling several of these safeguards simultaneously.

This is why Mukherjee insists cancer isn't one disease but hundreds, each defined by a different combination of malfunctioning genes and pathways, arising in different tissues — a fact that would take a century of research to fully appreciate and that explains why no single "cure for cancer" was ever realistic.

Takeaway: cancer is fundamentally a corruption of your own cells' growth controls, which is why it comes in hundreds of genetically distinct forms rather than one.