[... Cancer isn’t caused mechanically by outside agents...]
Really? Then why are we so afraid of asbestos and cigarettes?
Those fall in the category of environmental pollutants that can contribute to weakening a susceptible individual's immune system in such a way that a cancer can develop, but the other factors would have to be there, such as a genetic predisposition and possibly a triggering virus. Otherwise, 100% of people exposed for a long time to cigarettes or asbestos would get cancer. And we all know some old grandpas (mine died at 80 after smoking unfiltered roll-your-owns from the age of 7) who did not die of the same pollutant.