It's not so much hands that are a problem. If you walk around outside (and get chicken poo on your shoe), then walk around in the chick area (transferring that chicken poo to the ground there), then let the chicks out of the brooder for some playtime. Well, you can see how easy it is for them to contract coccidia. And salmonella is actually very rare in backyard flocks. Unless your birds are living in their own filth, it's not really a cause for concern. Coccidia, however, is everywhere and unavoidable. Chickens (and other animals) become resistant and develop immunity to the types of coccidia they are exposed to, rending those protozoa fairly harmless. Young chicks, however, have weak immune systems and can easily get overwhelmed.