Since vCJD appeared, scientists have been unable to agree on what the causative agent that could act on different animal species might be. One controversial theory identifies rogue proteins that have gone awry in the brain known as prions as the agent. Others suggest the disease may be the result of an autoimmune disease, a virus, a bacterial infection, or an agent lurking in the grass and hay that the ruminants eat. Results of recent research show compelling evidence that vCJD and certain other types of spongiform encephalopathy may be transmissible among animal species. However, the only spongiform encephalopathy known to be transmitted from animals to humans has been BSE, and the means of transmission remains unknown.
Many scientists maintain that no transmissible disease agent has yet been identified irrefutably.