I think it is real, in that there is what appears to be a legit biological mechanism for it to happen. I'm no biologist, though. In a nutshell, cancer seems to be the uncontrolled reproduction of a cell caused by faulty/damaged DNA. If nothing else kills you, cancer will eventually get you. Your DNA's telomeres can succumb to wear and tear over years and years of dividing, leading to a cancer cell. Radiation, Earth-borne or from outer freakin' space, can lead to a cancer cell. Oxidative stress can increase DNA mutations or cause DNA damage, leading to a cancer cell. It could be your lucky day, and a random DNA mutation gives you the gift of a cancer cell. But like I said, I'm no biologist, and modern biology could be a CIA psyop/medical scam for all I know.