The major disease most strongly associated with a known, avoidable risk factor that I can think of is lung cancer. And Wikipedia tells me that

> The vast majority (85%) of cases of lung cancer are due to long-term tobacco smoking.[4] About 10–15% of cases occur in people who have never smoked.[14] These cases are often caused by a combination of genetic factors and exposure to radon gas, asbestos, second-hand smoke, or other forms of air pollution.

So even for lung cancer, a significant minority of sufferers have never smoked. Genetic factors are obviously out of one's control, and the others mentioned can be pretty damn hard to avoid, especially if you're poor and/or born in the wrong place.