system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does rabies work across different species and manage to cause the same symptoms?
gpt
Rabies is REALLY old (its on every continent and pre-dates colonization). Aristotle wrote about it, and Mesopotamians had fines for owners of rabid dogs. In the Americas, there is fossil evidence of rabies in bats and skunks - but that particular strain wasn't as infectious for dogs. Viruses that are really old either go extinct or figure out how to be optimally infectious. Rabies is infectious enough to spread, since it can infect any mammal it maintains a reservoir, but it's way of spreading (feces,spit, blood) keep it from becoming an epidemic - so mammals don't develop immunity.

Rabies spreads so easily in mammals because of its shape and target - it goes after the spinal chord. This is also why symptoms are pretty uniform (lethargy or aggression). The virus is attacking the "oldest" (evolutionarily speaking) parts of the brain. Things that are viewed as "older" in this way tend to work similarly across related animals, in most cases. Your fight/flight center of your brain falls into this. Mammals also have similar body temperature ideal for the virus, this is why birds, snakes, and fish can't get rabies, but also why possums don't generally get it (they have really low body temperatures).

As an unrelated, but fun fact, Louis Pasteur (the father of pasteurization and our understanding of germs and diseases) invented the vaccine that we still use today and is the only reliable treatment for rabies. He administered a largely untested vaccine (it had been successful in animals) to a kid who was bitten, and the kid survived.