Final answer:
Stanley Prusiner first identified scrapie in sheep as being caused by prions. His discovery, which won him the Nobel Prize, changed the understanding of infectious diseases, showing that they could also be caused by proteins without DNA or RNA.
Step-by-step explanation:
Stanley Prusiner first identified the disease scrapie, a fatal, degenerative disease in sheep, as being caused by prions. In 1982, Prusiner discovered that scrapie was transmitted by prions, which are infectious agents composed entirely of proteins. This was a significant discovery as it challenged the prevailing belief that all infectious agents must contain DNA or RNA. Prusiner's findings, although initially met with skepticism, were later validated and led to him being awarded the Nobel Prize in Physiology or Medicine in 1997.
Prions are responsible for various fatal neurodegenerative diseases, including Creutzfeldt-Jakob disease (CJD) in humans and bovine spongiform encephalopathy (BSE), also known as mad cow disease, in cattle. Prion diseases affect the brain, leading to its rapid degeneration and giving it a sponge-like appearance due to the formation of holes. Transmissible spongiform encephalopathies like CJD, kuru, and scrapie are untreatable and ultimately fatal.