As small and simple as viruses are, they dwarf another class of pathogens, viroids. These are tiny molecules of naked circular RNA that infect plants. Only several hundered nucleotides long, viroids do not encode proteins but can replicate in host plant cells, apparently using cellular enzymes. Some how, these RNA molecules can disrupt the metabolism of a plant cell and stunt the growth of the whole plant. Viroids do not affect animals (according to present discoveries) as they are not associated with protein that may help them to cross the protective cell membrane. But in plants they enters through stomata.

An important lesson from viroids is that a molecule can be an infectious agent that spreads a disease. But viroids are nucleic acid, whose ability to replicate is well known. More difficult to explain is the evidence for infectious proteins, called “Prions”. Prions appear to cause a number of degenerative brain diseases, including scrapie in sheep and the “mad cow disease”. How can a protein, which cannot replicate itself, be a transmissible pathogen? According to one hypothesis, a prion is a misfolded form of protein normally present in brain cells. When the prion gets into a cell containing the normal form of protein, the prion some how converts the normal protein to the prion version. In this way, prions might repeatedly trigger chain reactions that increase their numbers.