Contagious Equine Metritis, or CEM, is a bacterial infection that is transmitted during breeding, but may also be spread by humans and stable equipment, although humans are not known to contract the infection itself. North America is believed to be free of CEM, with the last known cases here occurring in the late 1970s. The disease is found in horses in Europe.
You won't be able to look up CEM in your grandfather's vet manuals; the disease has only been identified since the 1970s, when mare vaginal infections in the United Kingdom were linked to the gram-negative bacteria Taylorella equigenitalis, qv.
CEM in stallions is one of the main diseases tested for by the USDA in quarantine, and horses are not allowed into the US who test positive for the disease until they undergo treatment and test "clean". In stallions, there are no outward symptoms, and the disease may be transmitted via artificial insemination as well as by traditional breeding.
Infertility is one of the leading symptoms in mares, but symptoms vary in severity, usually beginning with a vaginal discharge followed by inflammation of the uterus and/or cervix. Once the acute stage of the disease is over, the mare continues to shed the bacteria.
Treatment includes antibiotics, although some strains apparently are resistant to streptomycin. Some countries in Europe have CEM procedures mandated by law, while others, including Austria, do not classify CEM as a reportable disease, which makes CEM such a frustrating problem in the realm of equine health.
The USDA's web site has an excellent information page on CEM.