From the L.A. Times:
As an outbreak of highly contagious horse herpes infects horses across Western states, leaving some horses dead and prompting event organizers to cancel competitions, a closer look at the virus causing all the trouble would seem in order.
But first, as Reuters reports: "Horses cannot infect humans but for the animals the symptoms of the virus include respiratory problems and hind-leg weakness, decreased coordination, nasal discharge and fever."
Nine strains of equine herpes virus have been identified; the strain in the news, EHV-1, is very common. By age 2, almost all horses have contracted the virus, usually from their mothers (dams), but the virus generally lies inactive, according to an information sheet from the U.S. Department of Agriculture.
For more, see: Horse herpes: The virus behind the outbreak