"Birds that migrate across continents tend to have a higher viral load than those that stay at home year round. But for some of those species, migrating might help to avoid the more virulent pathogens that can accumulate in one environment. Other animals might use this migratory escape method as well."
This is all fine and well... but considering human impact on the environment: "Better understanding these disease dynamics will be important in stemming potential pandemics. But migrations, like the animals themselves, are moving targets, influenced by human impacts to the physical and climatic landscape. With less available land and more impediments to movement—such as fragmented habitat—more animals might end up in a few close-quarters stopovers like Delaware Bay or curtail their migration, leading to fewer opportunities to escape local infection or leave the sick behind."
This is a good article with a decent bit of embedded links imo. Check it out I found it a good read.