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 are can cause crop loss. Viroid genomes are extremely small in size. They are about 80 times smaller than the smallest virus. The human pathogen (causes diseases in humans) hepatitis D virus is a defective RNA virus similar to viroids.