Epidemiology
These spontaneous disorders in humans are very rare, affecting only about one person per million worldwide each year. However, transmissible TSEs can reach epidemic proportions, as was seen in the UK BSE outbreak of the 80s and 90s. It is very hard to map the spread of the disease due to the difficulty of identifying individual strains of the prions. This means that if animals start to show the disease after an outbreak on a nearby farm, you cannot show that it is the same strain affecting both, suggesting transmission, or that the second outbreak came from a completely different source.
Read more about this topic: Transmissible Spongiform Encephalopathy