Transmissible Spongiform Encephalopathies (TSEs), viral types of organisms often called "slow viruses", cause progressive and fatal organic brain disease. Thus far, several types have been identified. Given the name due to the spongy appearance of the brain when examined microscopically, TSEs affect humans and/or animals, appear to have a species barrier (are most likely to affect members of the same species), and are primarily transferred through ingestion of--or direct exposure to--body tissue contaminated with the disease. In humans, symptoms resemble senile dementia/Alzheimer's disease with a much faster onset and progression. Just what causes the disease, for which there is no cure, remains uncertain.

Scrapie, found primarily in sheep and reported in many countries, was first recorded in British flocks in 1732. Although it occurs naturally in sheep and goats, it can be transmitted to certain other animals. Bovine spongiform encephalopathy (BSE), or "Mad Cow Disease...