When a horse goes from where it naturally stays to a new environment, it has not built up an immunity to the microbes in the new environment, and is very susceptible to becoming sick.
In fact, it is the most expensive horses, the top international racers and multi-million dollar sires and dams that are among the most-traveled horses. And the most exposed to new microbes against which they have no built-up immunity.
Younger horses, too, are particularly susceptible to picking up new bugs because they are often the animals moving around the most. Yearlings and two-year old thoroughbreds just beginning their careers live in constantly changing environments as they graduate from running beside their moms, to sales venues, to training centers, to the racetracks. The moves are generally accomplished by horse vans with different horses from different sites traveling in them. And, increasingly, horses – especially valuable ones -- travel overseas and back via air. Horses are always on the move, and always meeting new sources of viral trouble.