Viruses and prions are two types of infectious agents. Viruses are small microorganisms that infect other living organisms and consist of nucleic acid surrounded by a protein shell. They can cause oral diseases. Prions are proteinaceous particles that can self-replicate and induce neurodegenerative diseases like Creutzfeldt-Jakob disease in humans after a long incubation period. They are highly resistant to inactivation.

3. VIRUSES * One of the smallest forms of microorganisms and infects other forms of live: Animal --- Plants --- Bacteria * They can cause oral and orofacial disease - - produces oral signs of systemic infection and transmitted to patients and dental staff

4. Main Features: * Small size (10-100nm). * Genome: either DNA or RNA but never both- ss (single strand) or ds (double strand), linear or circular. * Metabolic activity: Obligate intracellular parasites.