There is a rare illness called mad cow disease. It is also called as bovine spongiform encephalopathy (BSE). This disease affects cow's nervous system causing a cow to act strangely. According to scientists, a person is at higher risk of getting a human form of the disease, CJD (Creutzfeldt-Jakob disease), when he/she eats BSE-infected meat. You have studied various disease-causing agents. Which agent do you think is responsible for this rare disease for which there is no treatment yet?