Every year about this time, we begin to hear warnings about the “flu”. The warnings flow from the colder weather, but sometimes we also hear warnings about the “bird flu”–a particular variety of “avian influenza” that’s slowly spreading across Asia and will spread around the world based largely on the fact that birds can fly.
In theory, the bird flu is most contagious on a global basis whenever some species of birds migrate from one nation or even continent to another in the Fall and back again in the Spring.
Once the migrating birds reach their Winter home or their Summer home, they stay more-or-less in that place. Other “local” birds may pick up whatever disease the migrating birds are carrying. Those diseases can ultimately reach the local people.