It is characterized by the contamination of the animal by viruses, they can be found in many organs of the horse according to each type of virus. Mainly viral infections is observed in the respiratory tract, in the digestive tract and in the reproductive tract of the horse. There are also those viruses on the skin, in the nervous system and also in the circulatory system. Viral infections have very different clinical signs and can have dramatic consequences in the horse up to the death of the animal. Common clinical signs include depression and fever.