No, Kent the agent in BSE, Kuru, Creutzfeldt-Jacob and related,is not really a virus.  In fact it is unknown, but is a proteinaceous particle (prion...so named by Dr.Stanley Pruisner).   The diseases themselves, are in fact called Slow Viruses, because of the long incubation period, but the title is somewhat misleading, since they are clearly not due to viral infection.   Also,the cause of Alzheimer dementia is different than these dementias, and is most likely (according to current knowledge), due to abnormal metabolism of Amyloid, and amyloid precursor protein. The biochemistry and genetics of this process is becoming much clearer in recent years.

The risk factor for infection is eating contaminated meat, but the highest concentration of the infection agent is in nervous tissue.  I once had a patient who developed Creutzfeldt-Jacob, and in trying to pin down the source, I asked he and his family if he hunted, and ate game meat or brain ( a delicacy for some).   As it turns out...he was Greek, and there is a Greek festival during which sheep or lamb brain was eaten (most of the younger folk in his family ate the meat, but he was the only person to consume brain).   The poor guy died not long after we made the diagnosis (biopsy proven), but fortunately did not suffer very long.   The family gave me a nice urn as a gift and which I still have, for the care he received.

Until Mad Cow...this disease was very rare (fortunately).  We will now inevitably see a "spike" in incidence within the next 10 years or so.   There will be no way ante morbitum to tell if one is infected, but if the tainted meat is eliminated immediately, and the practice of feeding cattle ruminants is discontinued, the illness should be preventable (at least it will return to rare incidence).  

Gene