Viroid

Viroids are the smallest infectious pathogens known. They consist solely of short strands of circular, single-stranded RNA without protein coats. They are mostly plant pathogens (plant diseases), some of which can cause crop loss. 

Viroid genomes are extremely small in size. They are about 80 times smaller than the smallest virus.Dasgupta M.K. Principles of Plant Pathology (New Delhi: Allied Publishers, 1988), p. 132 The human pathogen (causes diseases in humans) hepatitis D virus is a defective RNA virusA defective virus cannot make copies of itself outside of a living host cell similar to viroids.

Viroids were the first "sub-viral pathogens" discovered and named by Theodor Otto Diener.  He was a plant pathologist at the U.S Department of Agriculture's  Research Center in Beltsville, Maryland, in 1971. The first viroid to be identified was the Potato spindle tuber viroid (PSTVd). About 33 species have been identified.

 Related pages 
 Obelisk (biology), a viroid-like structure found in humans.

 References 

Category:Viruses