VIROIDS AND PRIONS There are infectious particles that are even simpler than viruses. They will contain only a nucleic acid like RNA or only a protein that can cause disease. The three different types of these infectious particles include viroids, virusoids, and prions. Viroids are simply segments of circular RNA that can self-replicate. The viroids can hijack the host’s replication machinery in order to replicate the RNA segments. There is no protein coat involved. An example of a disease that can cause this is the potato tuber spindle disease. There are several severe plant-related viroid diseases that can damage crops and kill the host plant. These will be passed in similar ways as happens with viruses. Virusoids are also sub-viral particles. These are single-stranded RNA molecules that are related to viroids but differ in that they require a coinfection with another helper virus. These types of infections are much less common than viral or viroid infections and mainly infect plants. There are no proteins associated with virusoids themselves. Hepatitis D or hepatitis delta is one of these satellite RNA segments that can only infect a human that has the helper virus hepatitis B in order to result in the hepatitis D infection. Prions are not nucleic acids but are proteins that can be infective. Prions are a misfolded protein that resembles a natural protein but instead of being active, it leads to plaques in the tissues. The main prion that has been studied is one that causes transmissible spongiform encephalopathy or mad cow disease. The end result is the killing of brain cells, which becomes sponge-like in appearance. Infected humans will have dementia and the rapid onset of death within months. Creutzfeldt-Jakob disease is an example of this type of infection, although there are others. There is no cure for these diseases as they cannot be killed off.
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