The diseases are mostly due our lifestyle. According to a research, one can reduce the likelyhood of getting cancer by 80% by eating healthy and being physically active.
I think "mostly" is a statement neither of us can prove. But cancer just happens, even to perfectly healthy people. It was a lot less common back when most people lived only 35 or even 55 years. Now, with plenty of people living till 75+, they have a lot more time to accumulate carcinogens and part of it really is simply luck.
Everybody good, plenty of slaves for my robot colony?