Rebecca Siegel still remembers the moment it started — a flash of data on her computer screen that stopped her cold.

It was 2008, and Siegel, then a young epidemiologist at the American Cancer Society, was preparing a report on colon cancer. Since regular colonoscopies became routine for people over 50 in the 1990s, colon cancer had been a point of pride in the gloomy world of cancer news. Tumors were caught sooner, and pre-cancerous polyps could be removed before they turned deadly.

On a whim, Siegel pulled up the data for people under 50. What she saw shocked her: More than 10,000 people in their 20s, 30s, and 40s had been diagnosed with colon cancer that year. The number was relatively small, but it had been climbing steadily for over a decade.

"This is big," Siegel remembers thinkin

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