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.
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.