Something religions don't consider; supposing their "intelligent designer" didn't design things for the benefit of humans? Maybe there's a god of bacteria?
Most ancient religions had competing gods sponsoring different animals, human pursuits, aspects of the weather... makes more sense to me, if there were in fact going to be gods, they would be in charge of different bits of the apparent chaos that is our world.
Genome.gov says over 6000 currently know disease or disorders.