The equine sarcoid is a skin tumour that affects horses, donkeys, mules and other equids (including the zebra) throughout the world. Around 3-8% of horses are affected by the condition but there are geographical differences in both the proportion of horses that are affected and the specific types of sarcoids those horses get.  In Australia for example there is a very different disease profile from the UK.

The disease is problematic because it is very unpredictable and also very difficult to treat. There is a close association with a virus that affects the skin of cattle (bovine papilloma virus). Whilst the disease can seriously affect the skin at any site on the body, it does not spread into the internal organs, so while many horses are badly affected, it seldom "kills" the horse. However, many horses lose value or are destroyed because of the disease.