genetics

A very large study of Nordic twins published recently in JAMA found that there's a significant link between their genetic makeup and their risk of getting certain cancers, with the strongest links involving melanoma and the prostate.
The evolution and spread of genetic disease is fascinating because genetic diseases (spread through inheritance only) really shouldn't exist. They are caused by having alleles (versions of a gene) that are detrimental to health and biological fitness, so they should not remain established in a population.