An informative article from The Horse magazine:
By Heather Smith Thomas
There’s a life-threatening disease horses can harbor in their bodies without showing any signs of illness. But under stress—even inapparent stress—the horse can disperse the virus with every cough or sneeze, exposing nearby equids to the pathogen. All of this can happen undetected until, perhaps, a horse in the same barn turns up with a fever or another begins showing neurologic signs.
This nightmarish scenario can mark the start of an equine herpesvirus-1 (EHV-1) outbreak, which most frequently occurs where horses congregate, such as at horse shows, trail rides, or barns with transient populations.[More]