I too have never understood it. I also dont understand people who let an animal that has no control over its sphincter to walk all around their home. This meaning guinea pigs, birds, chickens and the like. I've personally owned a rabbit, a rat, a gerbil and a hamster. Only the rat was allowed to walk around freely because I trained it to do its business in its cage. My problem is not with the idea of disease...its the poop. I see people all the time that just let pigeons, cockateils, and birds just fly around their home with NO diaper. Poop would be everywhere unless you followed that bird all around cleaning up behind it. I dont think I could live like that. Maybe someone with birds like this could explain how they deal with the poo problem?