Answers:
It evolves.
That is why we own a different liberal of flu every year. Actually, we usually own two or more kind every year.
Influenza A (the unpromising kind) is passed between man and animals, mostly birds and pigs. Birds move adjectives over the world, so they can spread it really capably. They spread it to domestic poultry that humans come within contact next to. Not adjectives bird or pig flu is contagious to humans, and vice versa.
But the bugs evolve. They can become contagious between species. They can translation so they are only plenty different from the one you have final year so that you enjoy no imperviousness to it when it comes around this year.
The bird flu that adjectives of the public strength agencies are concerned beside have be spreading around the world as birds migrate. Only a few humans enjoy gotten it, and they be adjectives ancestors working beside poultry. There is a nurse who apparently caught it from a lenient, though. If it evolves ample to spread efficiently from human to human, we are adjectives within big trouble, because humans hold virtually no imperviousness to it. Most of us own some imperviousness to mixed influenza virus types because we own have them, but this type have never be out at hand contained by any of our lifetime.
So the virus doesn't lay around dormant. It is other out in attendance moving and evolving.