Viruses
- Made up of proteins, nucleic acids, and sometimes lipids
- Can only reproduce by infecting a living host
- Don't use energy or make molecules
Prions
- Prions are forms of proteins that cause diseases
- Mad cow disease is caused by a prion
- They don't even have nucleic acids.
Viroids
- Viroids are non-coding strands of RNA that infect plants.
These are not considered to be alive because they don't meet some of the requirements for life. They are not cellular, cannot reproduce on their own, and prions don't even have nucleic acids or basic levels of organization.
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