Solution:Prions are a smallest proteinaceous infectious particle. Mycoplasma is a genus of bacteria that lack a cell wall around their cell membrane. Viroids are the smallest infectious pathogens known consisting solely of short strands of circular, single-stranded RNA without protein coats.
They are mostly plant pathogens, some of which are of economic importance. Rickettsia is a genus of non-motile, gram-negative, non-spore forming, highly pleomorphic bacteria. They have cell wall.