Why can someone with a strong immune system and no family history of a certain kind of cancer get cancer? What causes those gene mutations?

For example, I know a 23 year old with an acute form of leukemia. No family history of leukemia. She did all the right things and had a very strong immune system until she got leukemia, she wouldn’t even get colds. So we’re all confused. She also doesn’t and never has lived near any places with high radiation,... show more For example, I know a 23 year old with an acute form of leukemia. No family history of leukemia. She did all the right things and had a very strong immune system until she got leukemia, she wouldn’t even get colds. So we’re all confused. She also doesn’t and never has lived near any places with high radiation, she’s only had x-rays done at the dentist, and her jobs haven’t exposed her to any carcinogens.
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