Normally horses' immune systems keep the virus at bay, but stress and an increase in contact may break down defenses.
Although most adult horses are infected with the herpes virus, stress can cause the virus to become active and contagious.
The present horse herpes outbreak centers around a show horse event that was held in Utah about a month ago.
The outbreak has resulted in at least 84 confirmed cases of the disease in horses from 10 states so far.
Equine herpes, a highly contagious infection among horses that can be fatal, may spread when stressed out show horses come together for competitions, according to animal health experts.
That appears to have helped fuel the current equine herpes outbreak, which has killed at least 12 horses and sickened 72 others in 10 states so far. These states include Arizona, California, Colorado, Idaho, New Mexico, Nevada, Oklahoma, Oregon, Utah and Washington.
"Most adult horses are infected with the virus," Philip Johnson, a professor of equine internal medicine at the University of Missouri's College of Veterinary Medicine, told Discovery News. "Like most herpes viruses -- human and animal -- infection leads to a life-long association between the virus and the host. In most healthy horses most of the time, the host's immune system prevents the virus from going active and being especially contagious."