I think you have to go on a case by case basis-disease wise. For example, the number of children with down syndrome might increase because more women are having kids when they're 35 or over with the help of drugs, which increases the liklihood of having a child with down syndrome.

However, many incidences of diseases that come up in close knit communities might decrease because of technology. We can choose potential mates from all corners of the earth, and more and more people are migrating from country to country. genetic variation is a good thing evolutionally speaking. It kind of "dilutes" bad genes.