Sometimes called "Mad Cow Disease." A disease of cattle presumably caused by a virus or other unidentified entity that affects the brain and causes the cow to behave erratically. Prevalent in parts of Europe but not in the U.S., BSE is a contaminant that is undesirable in bovine sera. It is not known whether the causative agent can be filtered out, since the causative agent itself is not known. In humans, it is believed to cause Creutzfeld-Jacob, a disease affecting the nervous system.