Farm animals are transported throughout the country. This also applies to their pathogens.

The avian flu virus spreading through American dairy cows is likely traced to a single spillover event. Scientists believe the virus jumped from wild birds to livestock in the Texas Panhandle late last year. This spring, the virus known as H5N1 had traveled hundreds of miles or more, appearing on farms in Idaho, North Carolina and Michigan.

The virus did not travel those distances on its own. Instead, it hitched a ride with its hosts, the cows, and moved to new states as cattle were transported from the epicenter of the outbreak to farms across the country.

The transport of ...