Scrapie is a natural disease of sheep, but it can also be successfully transmitted between sheep by experimental inoculation. Although BSE is primarily a disease of cattle, it has also infected humans (causing vCJD) and, in addition, can be transmitted orally to sheep bringing concerns that BSE might naturally have infected the UK sheep population. Because of this, scrapie and BSE are being compared and studied in detail in sheep. PrP genotype controls sheep susceptibility and resistance to scrapie and to BSE, and deposition of the disease-associated PrP(Sc), used as a marker of infection, has the potential to act as a means of identifying BSE-infected animals and describing different pathogenesis mechanisms. Sheep orally dosed with BSE show signs of infection in their blood and this model is of major importance in the study of the safety of blood products for use with human beings.