At age 28 my brother was diagnosed with leukemia.
He took up long distance cycling, and running. He lived 9 normal years.
But when the cancer decided to strike, he was dead in about 4 months.
(Maybe if he had not exercised, it would have killed him in 1 or 2 months instead.)
https://www.youtube.com/watch?v=K05Db4WGxXg
I exercise everyday. I never get so much as a cold or flu, unless I stop.