Viruses are the most complex of these infectious agents. They consist of a protein coat (capsid) that encloses a core of genetic material (either DNA or RNA). Viruses can only replicate inside living cells, where they use the host cell's machinery to make copies of themselves.
Viroids are much smaller than viruses and consist only of a single strand of RNA. They lack a protein coat and do not encode any proteins. Viroids replicate by hijacking the host cell's RNA polymerase enzyme, which uses them as templates to make new viroid RNA molecules.
Prions are the most unusual of these infectious agents. They are not composed of nucleic acids, but rather of a type of protein called a prion protein. Prions cause disease by misfolding and causing normal prion proteins in the host to misfold as well. This accumulation of misfolded proteins leads to damage and dysfunction in the host's cells and tissues.
Despite their differences, viruses, viroids, and prions share some common characteristics:
* They are all infectious agents that can cause disease in humans, animals, and plants.
* They all lack the ability to reproduce independently.
* They all require a host cell to replicate.
Because of these characteristics, viruses, viroids, and prions are all considered to be parasites.
