There is a school of scientific thought that disease is a stepping stone in evolution. there is a some trade off and/or it slowly rids the gene pool of faulty genes.



That is true actually - back in the days when there was no chemo and cancer treatment, those people would have died and therefore the faulty gene would have been removed from the gene pool.

Nowadays that person would survive and have children.

Diabetes (type 1) was less prominant as well because there was no insulin so anyone with diabetes would have just died when they were babies, now the diabetes gene gets passed on.