The most difficult mi­cro­bi­ol­o­gy riddles

Viruses are by far the smallest, their whole "body" is a DNA or RNA code packed in a protein coat. Unlike the rest of the given, they are not living cells.

All known prion diseases affect the structure of the brain or other neural tissue and all are currently untreatable and universally fatal. This includes bovine spongiform encephalopathy (BSE, also known as "mad cow disease") in cattle and Creutzfeldt–Jakob disease (CJD) in humans.

A prion is a single protein in a misfolded form. The prion theory was developed to explain the discovery that the mysterious infectious agent causing the diseases scrapie and Creutzfeldt–Jakob disease resisted ionizing radiation. The data suggested that the infectious agent was too small to be a virus.

A prion is composed of protein in a misfolded form. When a prion enters a healthy organism, it induces existing, properly folded proteins to convert into the disease-associated, prion form; the prion acts as a template to guide the misfolding of more proteins into prion form.