Viroids and Prions differ from viruses in many important ways. Unlike viruses, viroids do not have protein capsids protecting their nucleic acid. Unlike viruses, prions do not contain nucleic acid.
Viroids destroy economically important plants. For example, viroids are the main cause of diseases that have killed millions of coconut palms in the Philippines.
Prions have been found in the brains of sheep, killed by a disease called Scrapie. Prions have also been found in the brains of cows that died from the infamous Mad Cow disease.