I never had or even heard about the flu when I used to live in Brasil. The moment my brother and I moved to the US and had to get a vaccine to get into the school, yes I stayed home for 2 weeks with the flu.
I know animal's immune system is way better then ours and to hear the viruses comes from them is kind of crazy. Now I know many will carry diseases, but from what I know they might be nature's own.
http://en.wikipedia.org/wiki/Antidote
http://en.wikipedia.org/wiki/Antivenom
If animals are used to get us those, then a little virus wouldn't phase them.