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Humans are awash in potential cancer-causing agents, and it is practically impossible to determine the cause of a given case. Cancer does not awaken all at once. Instead, an amalgam of carcinogens, genetic predispositions, social factors, lifestyle choices, natural aging and the random chance of spontaneous mutation sets it off, bit by bit, year by year, even decade by decade.

“By the time you see a tumor in a mammogram, it may be that that tumor has been growing for six years,” said Rachel Morello-Frosch, a professor of public health at the University of California, Berkeley. “It’s very hard to pinpoint what it is that initiated that process.”