Viroids:

• Sub-viral agents
• Smaller genome (circular ss- RNA)
• Protein free (no capsid), low molecular weight
• Plant pathogens
• Resistant to heat and organic solvents but sensitive to nucleases.

Prion:

• Virus like infectious agents
• Only protein and without nucleic acid
• Resistant to heat, UV-rays, and nucleases
• Neurodegenerative disease (human), mad cow disease in cattle.