Many cancers are caused by viruses. HPV causes cervical cancer, hepatitis C can cause liver cancer, Epstein-Barr virus can cause lymphomas. Viruses also cause Diabetes and are linked to MS.

I personally think that almost all cancers are caused by viruses, we just don't know why some people develop cancer and others only get the initial cold/flu. Yes, there are environmental chemicals that cause cancer, such as benzene and formaldehyde, but I think fewer people are exposed to those.