In March a farm worker who reported no contact with sick or dead birds, but who was in contact with dairy cattle, began showing symptoms in the eye and samples were collected by the regional health department to test for potential influenza A. Experts have now confirmed the first case of highly pathogenic avian influenza transmission from a mammal (dairy cow) to a human.
Mammals are not identical and there are a ton of diseases that do affect multiple species and a lot more that don’t.
Opossums rarely, if ever, get rabies. Bats tend to get diseases without suffering from them, but are great incubators for diseases to mutate so they can spread to other mammals. Feline lukemia is extremely contagious between cats, but has never spread to humans.
It is not that humans are special, but that diseases are not universally transmitted between species.
This is a big deal because the bird to cow to human transmission doesn’t have a precedent. No scientist who studies disease thinks transmission is impossible, since diseases can mutate.
You just picked a mammal with one of the most unique immune systems and one that is a completely different class of mammal of which very few exist tbf. Many diseases can only affect a very small handful of even quite related species though. There are diseases that affect some apes but not others.