Viroids are among the smallest infectious pathogens known, larger only than prions, which are misfolded proteins. Viroids consist solely of short strands of circular, single-stranded RNA without protein coats. They are mostly plant pathogens, some of which are of economic importance. Viroid genomes are extremely small in size, ranging from 246 to 467 nucleobases. In comparison, the genome of the smallest known viruses capable of causing an infection by themselves are around 2,000 nucleobases in size. The human pathogen hepatitis D virus is a defective RNA virus similar to viroids.