>>12635433>What is the evolutionary reason of parasites and leeches existing?Parasites require a living host in order to sustain their own lives/populations. If one organism is successful in a particular niche, a parasite need only adapt to that organism as a host to ensure its own survival, provided the fitness cost is not too deleterious. Parasites that attach themselves to the skin of an animal don't even need to evolve complex ways of eluding the immune system of their host, they just need to have a strong, well-placed bite. Seems like a good idea to me.
