Viroids are the smallest infectious pathogens which are composed of circular single-stranded RNA without a protein coat. Viroids were first discovered and given this name by Theodor Otto Diener (1971) who was a plant pathologist works at Agricultural Research Centre in Maryland. The first viroid discovered was the Potato spindle tuber viroid (PsTVd) which cause disease in potato and some other solanaceae plants. D.J. Ivanowski was the discoverer of the virus.