Organisms Infectious agents
I Domain B^S^Ta Rua® I Viruses Viroids Prions
Prokaryotes Eukaryotes (unicellular)
Algae Protozoa Fungi Helminths
(unicellular or (unicellular) (unicellular or (multicellular multicellular) multicellular) parasites)
Figure 1.10 The Size of a Viroid Compared with a Molecule of DNA from a Virus that Infects Bacteria (T7) The red arrows point to the potato spindle tuber viroids (PSTV); the other arrow points to T7 DNA.
Prions are very unusual agents that are responsible for at least six neurodegenerative diseases in humans and animals; these are always fatal. Prions appear to be only protein, without any nucleic acid (figure 1.11). Although unlikely, it is possible that another agent that is very difficult to isolate might also be involved in causing the neurodegenerative diseases.
Figure 1.11 Prion Prions isolated from the brain of a scrapie-infected hamster. This neurodegenerative disease is caused by a prion.
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