"Mad Cow Disease" describes the condition in cattle caused by a misfolded protein, called a prion. Humans (and other animals) can also develop this disease when they ingest food made from products derived from cows with the disease. It is actually called bovine spongiform encephalopathy (BSE) in cows and variant Creutzfeldt-Jakob disease in humans (vCJD).
Only a small amount of the pathology surrounding Mad Cow is understood. It is rare for humans to get the disease since the FDA has strict bans on the use of certain cow parts in animal feed (that are most prone to harvesting the harmful prion), but any human who is unfortunate enough to get the disease will die after a long incubation period. Fatalities from the disease in both humans and cows are due to degeneration of brain tissue. Clinical features of sufferers include psychological abnormalities, vertigo, immobility and an inability to communicate (World Health Organization).
Currently, there is no clear method for diagnosing BSE in cows or vCJD in humans. Visualizing deteriorated brain tissue through a microscope after a death from either form of the disease is the only way to know for sure. There are no therapeutic options or preventative measures for BSE/vCJD (FDA).