Although people seem to understand about bacteria, viruses, etc., concerning human health, they act as if these things do not exist when it comes to birds. Since they can't see them, they assume they aren't there. They seem to feel that nothing bad will happen to them. One of the biggest downfalls of some owners and breeders is that they feel they can tell a sick bird just by looking at him. I wish they could but unfortunately when a bird shows signs of illness, he is very, very ill.
Birds are food for many animals.
In the wild, if a bird appears ill, it will attract predators. Fellow flock members may pick on the sick bird or chase him from the flock. Instinctively, the bird knows that he will not survive if he shows any signs of illness. He will try to act normal for as long as possible. Recently a breeder purchased a pair of new birds and immediately placed them with her other birds.