Earlier quoted context omitted.
Half of cancers in the developed world are due to lifestyle; so how you live has a huge impact in you chances of developing cancer. People often overlook other factors that increase your chances of developing cancer such as obesity, exposure to viruses, immunodeficiency, background radiation etc. Humans are in quite an unlucky position when it comes to developing cancer, due to our relatively small size and long life…
One other major factor people also overlook is genetics - not that you can do anything about it, but if you have cases of cancer in your family, you should probably be extra careful. Plus the obvious things like fair-skinned people needing to be more careful about skin cancer etc.
You won't always know there was cancer, though: How often do you hear about Grandpa's prostate or that mole your cousin had removed?