What ecological reason could be found for diseases as a population control?
I have provided several thought on the subject to help you answer your questions.
Each environment has a "carrying capacity," which is the largest number of any given species that the habitat can support. Once a population has exceeded that capacity, "survival of the fittest" behavior occurs. For example, if 100 birds are living in an area where there is only enough to food to feed 75 birds, then the other 25 birds will either move away or die, to put it in simplistic terms. Now, in reality, as you can imagine, conditions might be such that 95 of the birds have food, but less than they really need. Their bodies are not performing at peak condition, and they are more susceptible to sickness and disease. Eventually, the sickly become sicker and the remaining birds succeed. When organisms are competing for resources, the most fit win.
Of course, as you can see from "food webs," when something happens to one population, other populations are affected. For example, when environmental conditions are such that the mosquito population increases, ...
This solution explains how disease controls the size of a population.