This document discusses viroids, virusoids, and prions - three types of acellular infectious agents. It notes that viroids are small, circular RNA molecules that can self-replicate and cause diseases in plants. Virusoids are subviral RNAs that require a helper virus to replicate and infect plants. Prions are misfolded protein particles that can induce normal proteins to also misfold, leading to neurodegenerative diseases like Creutzfeldt-Jakob disease in humans and scrapie in sheep. Unlike viruses and viroids, prions contain no nucleic acid and are extremely resistant to destruction.