Viroids are plant pathogens that consist of a short stretch (a few hundred nucleobases) of highly complementary, circular, single-stranded RNA wthout the protein coat that is typical for viruses. The smallest discovered is a 220 nucleobase scRNA (small cytoplasmic RNA) associated with the rice yellow mottle sobemovirus (RYMV). In comparison, the genome of the smallest known viruses capable of causing an infection by themselves are around 2 kilobases in size. The human pathogen hepatitis D issimilar to viroids.

A virus is a small infectious agentthat can replicate only inside the living cells of organisms. Viruses infect all types of organisms, from animals and plants to bacteria and archaea.