I will mention this Gaia theory again. It basically says that beside evolution of organisms there has been evolution of the Gaia "superorganism". The Gaia takes care of healthy environment. Does Gaia also have methods to deal with occasional parasites?

Gaia theory is criticized along the lines that individual organisms or species cannot cooperate since "altruism" is not beneficial to them; organisms waste resourses only for themselves. But if the uncompromisingly selfish behaviour would be optimal, then all species would behave like locust:

Very few species have the luxury of eating up everything they come accross. Most species probably have instinctive or genetic stops not to destroy their own environment. After all, surviving throughout long millenia means not only overcoming stuggle to establish your own niche, but also surviving your own success. The Earth had seen many species that "discovered" unbounded success, and perhaps many of them learned to control their vitality so to keep livable environment.