Final answer:
Prions are infectious protein agents without nucleic acids, smaller than viruses, that can cause serious progressive diseases by converting normal proteins into misfolded, deadly forms.
Step-by-step explanation:
Prions are infectious agents composed entirely of proteins, specifically misfolded proteins that replicate by converting their normal counterparts into the disease-causing form. Unlike viruses, prions do not contain any nucleic acids such as DNA or RNA. The normal protein form is known as PrPC, and the infectious, misfolded form is called PrPSC. The misfolding of these proteins contributes to several progressive brain disorders, including mad cow disease and Creutzfeldt-Jakob disease. Infectious prions seem to produce their deadly effects by duplicating their shapes and accumulating in tissues. This concept was once considered impossible, but Nobel Prize-winning biologist Stanley Prusiner provided convincing evidence of their existence.