The infectious protein that causes diseases such as mad cow disease is called a prion. The word describes an infectious agent made primarily of misfolded protein, without the DNA or RNA genomes found in bacteria and viruses.
In bovine spongiform encephalopathy, or mad cow disease, an abnormal form of the prion protein can induce normally folded copies of the same protein to change shape. These misfolded proteins accumulate in nervous tissue, damage neurons, and produce the characteristic sponge-like degeneration associated with transmissible spongiform encephalopathies. The disease is fatal in cattle, and exposure to contaminated tissue has been linked to variant Creutzfeldt–Jakob disease in humans.
Prions are unusual because they are highly resistant to ordinary disinfection, cooking temperatures, and radiation compared with many conventional pathogens. The concept was controversial for decades because it challenged the assumption that infectious agents must contain nucleic acid. Stanley Prusiner popularized the term “prion” and received the 1997 Nobel Prize in Physiology or Medicine for research on these agents. Related prion diseases include scrapie, kuru, and chronic wasting disease.