Every May, a herd of viruses thunders through East Africa in one of the greatest migrations in the natural world.

For a few months, 1.3 million wildebeest head north through the Serengeti in search of food and water. There are so many of them that their lines can stretch from one horizon to the next. And almost every one of these animals is infected by a virus called acelaphine herpesvirus 1 (AlHV-1).

Wildebeest calves pick up the virus from their mothers within their first few months of life, and carry the infection until they die. The virus causes no symptoms, and doesn’t harm the wildebeest. But domestic cows aren’t so fortunate. If they contract the infection, they develop a disease called