Step 1: Concept Overview:
The question requires the definition of a prion, an acellular infectious agent.
Step 2: In-Depth Analysis:
Prions: These are infectious agents made entirely of misfolded protein, lacking any genetic material (DNA or RNA). Prions cause normal proteins of the same type to misfold, leading to aggregation and fatal neurodegenerative diseases such as Creutzfeldt-Jakob disease in humans and bovine spongiform encephalopathy (mad cow disease) in cattle. They are infectious proteinaceous agents.
DNA without protein coat: No known infectious agent matches this description.
RNA without protein coat: This defines a viroid, an infectious plant pathogen.
Protozoans: These are single-celled eukaryotic organisms such as Amoeba or Plasmodium, and are cellular, not acellular.
Step 3: Conclusion:
Prions are infectious proteinaceous agents.