A viroid (an infectious RNA molecule) is similar to a virus but not quite the same thing. It's smaller than a virus and has no capsid. A viroid is a coiled, "naked" RNA molecule that can affect a cell. Although RNA (unlike DNA) is single-stranded, the RNA in a viroid coils around itself to become double-stranded for strength. Its claim to fame is its ability to infect plant life. Although it has no protein of its own, a viroid's RNA affects a plant's ability to produce essential proteins.
Discovered by scientists in the 1970s, viroids affect crops like tomatoes, potatoes and fruit trees. They don't cause human disease, but they can cause crop failure, which means a huge loss of revenue for the agriculture industry each year [source: Microbe World].
Viroids typically spread via seeds or pollen. An infected plant might rub up against an uninfected neighbor, an insect could feed on different plants and spread the viroid around, or farm tools might distribute the infection throughout an entire field.
Prions (infectious protein particles) have neither DNA nor RNA to transmit infection. Not much is known about them, but we do know that a prion is an abnormal or mutated form of a usually harmless protein. When this normal protein becomes a prion, it becomes dangerous and can cause several mostly fatal neurodegenerative brain diseases. The most well-known disease caused by prions is mad cow disease (the human form is Creutzfeldt-Jakob disease).
Once prions enter the brain, they force the normal cellular proteins to begin folding into abnormal shapes. This destroys the neurons and eventually leads the brain to become riddled with holes. This spongelike brain consistency is where the medical name for mad cow disease comes from — bovine spongiform encephalopathy (BSE). Prions also cause the condition "scrapie," a degenerative disease affecting the nervous system, in sheep and goats.
Prions can cause a few human diseases, although they're extremely rare. One disease, kuru, is associated with cannibalism. Some scientists believe that prions may also have a role in Alzheimer's disease [source: Microbe World]. There is no treatment for any prion-caused disease.
You cannot destroy prions by sterilization or by cooking (although incineration works). Prions are resistant to heat, radiation and chemicals, so they are almost unstoppable. The best way to avoid spreading prions in humans and animals is to prevent brain and spinal cord tissue from getting into the food supply or onto medical equipment. Our research even turned up a novel called "The Zombie Autopsies" that imagines a zombie apocalypse caused by prions released by scientists [source: Stone]. The takeaway here is that although prions are small, they are mighty.
To wrap up, viruses, viroids, and prions all create havoc if they enter your body (or a plant or animal). They all have the same goal — taking over a living cell the way a pirate might take over a ship — but the way they achieve that goal is slightly different for each.