The chief difference between prions and viruses is that, while both cause disease, prions have no genetic material of their own. According to Merck, prions also differ from viruses in that prion-caused conditions are always fatal. Most viral illnesses can be survived. Prions tend to strike at the nervous system more often than at any other part of the body.
Viruses reproduce by injecting genetic material into living cells, which are then taken over and used to manufacture more viruses. Prions, unlike any living pathogen, have no genes. As described by Merck, certain prions, such as those responsible for Creutzfeldt-Jakob disease and fatal insomnia, are created by the abnormal folding of otherwise normal proteins found in every cell of the body. These abnormal proteins are chemically identical to their non-pathogenic form, but their abnormal structure alters their function in the cell. This often causes bubbles to form in brain cells and leads to the gradual death of nerve tissue in the central nervous system. This process is always fatal and cannot be cured. Unlike viruses, prions do not have to be contracted from an infected source, though they sometimes are, and can sometimes form spontaneously in the human body for reasons that are poorly understood.