Viriods are small circular RNA molecules without a protein coat that infect plants and animals. They replicate by hijacking the host's machinery and can cause diseases like potato spindle tuber disease in plants and Hepatitis D in humans. Prions are infectious protein particles that cause neurodegenerative diseases by changing the folding of normal host proteins. Examples include scrapie in sheep and mad cow disease in cattle. They are transmitted through ingestion and cause diseases by triggering apoptosis in the brain.