Assessment of the sero-prevelence of EHV-1 and/or EHV-4 in equine populations shows that antibodies to the viruses are detectable in most adult horses including other equine species.  EHV-1 and EHV-4 are characterized by the following features:
1)    Ubiquitous infection of young, susceptible horses
2)    High prevalence of latently infected carrier horses
3)    Re-occurring episodes of infectious virus shedding from such carrier animals, with or with out clinical signs,  that allow transmission of virus to new generations of equids
Quick reference numbers:

Distribution: Worldwide
% latently infected: 60-70%
Infection rate: ~100%
Incubation period:  1-10 days
Infective period:  14-21 days from nasal secretions after cessation of fever, up to 42 days on certain fomites (horse hair)
*Morbidity: 40%
*Mortality:  90%

* associated with outbreaks of Equine Herpesvirus 1:  Myeloencephalopathy