Viruses may be referred to often as the smallest infectious things. But there are some smaller contenders. Some of the agents of plant disease lack even a viral coat and are merely small strings of plain, or "naked," RNA. These particles are called viroids. They are believed to be a more primitive version of ordinary viruses.
Scientists in the 1970s began discovering even simpler and smaller virus-like organisms that can cause disease.
Viroids contain only RNA, but lack an envelope and capsid. Agricultural researchers found they caused problems in potatoes, tomatoes, and some fruit trees, and recently a viroid has been linked to hepatitis D.
But maybe viroids aren't the smallest infectious agents all.
Do you recall hearing about Mad Cow Disease? This is an ailment that affects the animals' brains and is also called bovine spongiform encephalopathy <boh-vine sponge-ee-form en-sef-uh-la-puth-ee> because it makes the brain appear holey, like a sponge. There is a human form of this disease called Creutzfeldt-Jakob <kroits-feld ya-cob> disease. Some scientists now believe these brain illnesses are among a few diseases caused by an infectious agents called prions <pree-ons>. Prions are not even DNA or RNA, but simply proteins. They are thought to be misshapen or abnormal versions of proteins normally found in animals or people. Very little is known about prions. Scientists suggest that they spread when a prion comes into contact with the normal version of the protein and causes the normal protein to change shape and become a prion, too.
Unlike other infectious agents such as bacteria, viruses, and viroids, prions do not have the nucleic acids DNA or RNA. Prions are proteins that have the ability to transmit diseases, a finding that defied scientific expectations. There is still much debate about how they work, but scientists think these rogue proteins direct the host to create abnormal proteins that can cause serious neurological disease in animals and humans.
Prions are blamed for scrapie in sheep, and bovine spongiform encephalopathy (“Mad Cow Disease”) in cattle, and its human variant Creutzfeldt-Jakob disease. Some scientists suspect that prions may be responsible for Alzheimer’s disease.