Equine Viral Arteritis
There are a few Friesian stallions that are currently positive for EVA. They can be positive in blood, and/or Semen. It's important to know about because one of the side affects it that it can cause abortions in mares or death in young foals. That is the part I'm going to focus on. Symptoms of an infected horse are in this article HERE
The virus is carried by the stallion, usually for life once they have it. It lives in Frozen, chilled and fresh semen. Mares that are inseminated with the affected stallion (or horses exposed to the + semen) will shed the virus for about 21 days after contact with the affected semen. They will spread it through respiratory secretions to any horse within range.
Unvaccinated mares in early term are likely to abort the foal. A late term pregnant mare may abort or may give birth to an infected foal that shows symptoms of pneumonia.
There is a vaccination for mares. They should be tested for their existing resistance (serology), then vaccinated appropriately. After being vaccinated they should be isolated for the 21 days/3 weeks that they are shedding the virus. The vaccine is not for pregnant mares, it's for pre insemination.
Colts can be vaccinated after 6mo of age, but this may affect their ability to get into other countries. I know Australia will not allow vaccinated stallions into the country or positive stallions in (as was in 2011, may change in the future)
For Embryo transfer it's recommended that the recipient also be tested and vaccinated if the stallion is EVA +.