Okay, this one is very NSFW - because contagious equine metritis is an equine venereal disease.
It's transmitted sexually (including by on farm artificial insemination, where workers may touch both animals with their hands or instruments).
Stallions can carry CEM without any symptoms - sometimes for years. In mares, the disease causes inflammation of the uterus and discharge from the vulva. Some mares may be asymptomatic and carry the bacteria for, well, months.
Mares are made temporarily infertile by the disease, although if they're already pregnant it rarely causes miscarriage. Unfortunately, it can take months for a mare to recover, and there's no good treatment for the uterine inflammation. On top of that, only mares show testable antibiotics.
External cleaning of the genitals with detergent and antibiotic ointment for several days is, however, effective at removing the bacteria from the outside (and is obviously the treatment for carrier stallions).
The disease exists in modern Europe but not in the United States (except for occasional outbreaks associated with imported stock).