How do diseases affect natural selection

Things die and the predators of those things don’t have food and they die or they change their diet to another easy prey.

The animals with the greater immune system will automatically be the next generation because diseases will nock out the ones with the weak, also, certain animals will become more immune as a few of them get the disease, live through it, have kids, the ones who couldn't fight the disease dies, and the next generations kids get the disease and become even more immune as they conquer it, etc. hope this helped, i know i was kind of everywhere...