 So they're standing more, more stress on their feet, switching their tail, just general annoyance factor. And so it's one more thing they're doing when they could be eating. Because flies are found anywhere, they're known to walk through some pretty nasty stuff that's likely going to be carrying some kind of bacteria or germs. Dr. Pertsborn talks about how pink eye, for example, is one of those nasty diseases flies spread very easily through a herd.