How do people develop cancer?

Multistep process: Cancer cells grow without being subject to the usual regulation that controls the growth of normal cells. People have different genetic susceptibility to cancer. Cancer is a multistage process that may be initiated by environmental factors, e.g. Smoking, drinking, pollutants in air and water, hormones in the body, radiation, infections etc. Lack of immune response allows cancer to progress.

Many answers: This question is very broad and we do not know in the majority of people. Certain exposures to cancer causing agents, and genetic predispositions are just 2 of the known causes.

2 hits hypothesis: Knudson suggested that multiple "hits" to dna were necessary to cause cancer. In inherited cancer, the first insult was inherited in the dna, and any second insult would rapidly lead to cancer. In non-inherited cancers, two "hits" had to take place before a tumor could develop, explaining the age difference.