The agent causing Mad Cow Disease has been linked through epidemiology studies (research that follows the causes and distribution of the disease among different populations) and laboratory research to a similar disease in humans that also damages that central nervous system. Scientists hypothesize that humans contract a form of Mad Cow Disease by consuming parts of the nervous system from infected cattle. The human form is called variant Creutzfeldt-Jakob Disease (vCJD), which is closely related to Creutzfeldt-Jakob Disease (CJD).

Variant Creutzfeldt-Jakob Disease (vCJD), is a fatal disease of the nervous system in humans. In the United States,the average age of patients with vCJD is 26 years, as opposed to 68 years for those who contract Creutzfeldt-Jakob Disease (CJD). This age discrepancy led researchers to look carefully at the younger patients and to uncover the connection between vCJD and Mad Cow Disease. On the basis of observable symptoms, probable cases can be diagnosed. However,  confirmation of vCJD cases requires examination of patients' brain tissue after death.

Prion diseases attack nervous system tissue. Prions have been associated with Bovine Spongiform Encephalopathy (BSE), Creutzfeldt-Jakob Disease (CJD), and variant Creutzfeldt-Jakob Disease (vCJD). Neither viruses nor bacteria, prions are thought to be proteins with abnormal shapes. Prions enter the body when people eat beef products that contain infected nervous system tissue. Once in the body, prions are believed to convert normal proteins in the nervous system into the abnormal prion shape.

Prions duplicate by attaching themselves to, and changing the structure of, normal proteins. It is believed that when a person eats the brain, spinal cord or other nervous system tissue from an infected animal, prions are absorbed into the body, where they slowly begin to transform normal proteins, eventually leading to fatal damage to the nervous system.