While I agree that it is important to find causes and thereby find means of prevention it is also important not to go nuts and just look for external factors.

Some people have better DNA repair and immune systems than others. Fact is also that DNA damage has a chance element to it. It may or may not happen and it may happen in one location or another where it may have dire consequences.
Even if you eliminate ALL external factors, admittedly a good thing! the possibility to get mutations and develop cancers will not be zero.

M

Ahh Brian just beat me to it!