Prions represent one of the most revolutionary and initially controversial concepts in all of biology — the idea that a protein molecule alone, without any nucleic acid, can be infectious and replicate. This concept challenged the central dogma of molecular biology (DNA→RNA→Protein) and required entirely new thinking about how biological information can be transmitted. The prion concept was proposed by Stanley Prusiner in 1982, who coined the term "prion" (Proteinaceous Infectious Particle) based on his research on scrapie (a prion disease of sheep). Prusiner's proposal that proteins alone could be infectious was met with significant scepticism and controversy, but the weight of experimental evidence eventually confirmed his hypothesis, and Prusiner was awarded the Nobel Prize in Physiology or Medicine in 1997 for his discovery of prions. The molecular mechanism involves a normally folded cellular protein (PrPC — C for cellular) that can be converted to an abnormally folded form (PrPSc — Sc for scrapie) by contact with already-misfolded PrPSc molecules, in a self-propagating chain reaction that progressively converts more normal protein to the pathogenic form and leads to accumulation of insoluble protein aggregates in nervous tissue.

BSE was first identified in cattle in the United Kingdom in 1986 and subsequently developed into one of the largest epidemic of a prion disease ever recorded, affecting over 180,000 confirmed cattle in the UK alone (with estimates suggesting millions were exposed). The epidemic arose from the practice of feeding cattle protein supplements derived from rendered animal carcasses (meat and bone meal, MBM) — a common practice to increase protein content of cattle feed — without recognising that prions in neural tissue from infected animals could survive the rendering process and transmit BSE to consuming cattle. Control measures including the ban on feeding ruminant protein to ruminants, culling of affected and at-risk animals, and removal of specified risk materials (brain, spinal cord, and other nervous system tissue) from the food chain eventually brought the epidemic under control. The BSE epidemic had enormous economic, social, and political consequences, including destruction of the UK beef export industry, severe impacts on farming communities, consumer crises of confidence in food safety, and significant changes to EU and global food safety regulations.

The BSE epidemic had a human health dimension that emerged as variant Creutzfeldt-Jakob disease (vCJD) — a novel form of human prion disease first identified in the UK in 1996 among unusually young patients (classical CJD typically affects people over 60, while vCJD patients were much younger, some in their teens). Epidemiological evidence strongly suggested vCJD was caused by exposure to BSE-contaminated beef products, confirmed by experimental transmission studies showing that vCJD prions were identical to BSE prions but distinct from classical CJD prions. By 2023, approximately 232 vCJD cases had been confirmed globally (predominantly in the UK), with the epidemic appearing largely contained following food safety interventions. Other human prion diseases include: Sporadic CJD (most common, ~85% of CJD cases, occurring randomly, cause unknown), Familial/genetic CJD (caused by inherited mutations in the PRNP gene encoding the prion protein, ~15% of CJD), Iatrogenic CJD (transmitted through medical procedures — historically through cadaveric human growth hormone injections, dura mater grafts, and neurosurgical instruments), and Kuru (now largely extinct, previously endemic among the Fore people of Papua New Guinea, transmitted through ritual cannibalistic consumption of deceased relatives including their brain tissue).

Viroids and prions are both simpler than viruses, causing infectious diseases without having the complete virus structure, but they are entirely different entities: Viroids: the smallest known biological pathogens, consisting solely of short (250-400 nucleotide), circular, single-stranded RNA molecules with no protein coat. They infect only plants and replicate within plant cells using plant RNA polymerase. Viroids do not encode any proteins — they function entirely through their RNA secondary structure and by hijacking host cell RNA processing machinery. Important viroid diseases include Potato Spindle Tuber Viroid (PSTVd), Chrysanthemum Stunt Viroid, and Coconut Cadang-Cadang Viroid (causing devastating disease in coconut palms in the Philippines). Prions: misfolded proteins with no nucleic acid component, infecting animals and humans, causing invariably fatal neurodegenerative diseases characterised by spongy degeneration of brain tissue (spongiform encephalopathy). The key distinctions for examination purposes: viroids = RNA only, plant pathogens; prions = protein only, animal/human pathogens.