Why are diseases more communicable within a species (e.g. human-to-human) than intraspecies (e.g. bird-to-human)?

Question

Why are diseases more communicable within a species (e.g. human-to-human) than intraspecies (e.g. bird-to-human)?

In: Biology

Answer 1

both humans and birds have immune systems designed to kill off invaders. So, to be a successful invader, you need to adapt to sneak past the immune defenses. Fortunately, for humans, most tricks that work to get past a bird’s defenses don’t work on a human.

Sadly, bats and monkeys are closer to humans, and so we tend to be susceptible to a few of their diseases.