Viruses, prions and viroids

Viruses- 

a small infectious agent that replicates only inside the living cells of other organisms. Viruses can infect all types of life forms, from animals and plants to bacteria and archaea. Virus particles (known as virions) consist of two or three parts: 

1. The genetic material made from either DNA or RNA, longmolecules that carry genetic information
2. A protein coat that protects these genes; and in some cases
3. An envelope of lipids that surrounds the protein coat when they are outside a cell. 

A virion is not the same as a virus, however. The infected cell is the virus, while the virus particles are ‘spores’ or reproductive forms. 

• The shapes of viruses range from simple helical and icosahedralforms to more complex structures. The average virus is about one one-hundredth the size of the average bacterium. Most viruses are too small to be seen directly with an optical microscope.
• Viruses are considered by some to be a life form, because they carry genetic material, reproduce, and evolve through natural selection. However they lack key characteristics (such as cell structure) that are generally considered necessary to count as life. Because they possess some but not all such qualities, viruses have been described as "organisms at the edge of life."
• They can be spread in many ways, through what are called vectors. Once infected, they provoke an immune response that usually eliminates the virus. Immune responses can also be produced by vaccines, which confer an artificially acquired immunity to the specific viral infection. Antibiotics have no effect on viruses. 

 - Examples:

1. Ebola
2. Rhino Virus - common cold
3. HIV virus - AIDs

Prions- 

an infectious agent composed of protein in a misfolded form. It is an abnormal form of a normally harmless protein found in the brain that is responsible for a variety of fatal neurodegenerative diseases of both animals and humans. 

• It is believed to be the smallest infectious particle
• A prion is neither bacterial nor fungal nor viral and contains no genetic material

- Examples:

1. Mad cow disease (Bovine spongiform encephalopathy)
2. Creutzfeldt-Jakob disease
3. Fatal familial insomnia
4. Kuru

Viroids- 

Viroids are infectious agents composed exclusively of a single piece of circular single stranded RNA which has some double-stranded regions. Because of their simplified structures both prions and viroids are sometimes called subviral particles. Viroids mainly cause plant diseases but have recently been reported to cause a human disease.

• Viroid genomes are extremely small in size, ranging from 246 to 467 nucleotides (nt), and consisting of fewer than 10,000 atoms.
• Essentially, a viroid is a short, single strand of RNA without a protein coat.

- Examples

1. Hepatitis D (only enters a human liver cell if it is enclosed in a capsid that contains a binding protein. It obtains this from the hepatitis B virus. The delta agent then enters the blood stream and can be transmitted via blood or serum transfusions)
2. Potato spindle tuber (the first viroid to be identified, natural hosts are potatoes and tomatoes)
3. Citrus exocortis (a disease of citrus plants that causes stunted growth and reduced yields in affected plants)

Are they alive?

Viruses, prions and viroids may behave like living things, but they are merely molecules that can affect a living cell. They are not considered "alive" because they are incapable of carrying out all the life processes (see the characteristics necessary for life). More specifically, they are not alive because they need a host to survive. 

Some might make the argument that they are alive since they feed and reproduce, and because viroids and viruses have RNA. However, while they are complex organisms that imitate life in many ways, they are still not alive.

Viruses, prions and viroids are just made up of proteins and nucleic acid, which are equivalent to a small component of a cell. Their behavior is far less complex than that of a bacterium, for instance. And the same goes for life cycles. 

Besides all the characteristics necessary for life, at the end of the day being deemed "living" comes down to complexity, and these structures can't shine a light to even the smallest bacterium.