Why does the coughing and sneezing worry Jerry?

From the text:

These sights disturbed him deeply because they told him that the agent had gone all through the building. He could see some of the animals coughing and sneezing, as if they had the flu. He wondered if he was seeing a mutant form of Ebola—a kind of airborne Ebola flu. He shrank from the idea and tried to turn his mind away from it, for it was too awful to contemplate.