His family's health history
Michael had cancer.
Imdb says that his sister died before Michael was born and his dad died when Michael was 11.
Bad genes? Bad family health? Coincidence?
Genes for sure. Alan Rickman's dad died when he was 8. Don't know why though.
Ken Burns has a good documentary series called "Cancer, the emperor of all maladies". It's on Netflix now. It talks about genetics being possibly one of many causes of cancers. Michael's one of the lucky ones who caught it before it was too late.