And You Thought Viruses Were Small!

Only the fraction of the size of a bacteria, viruses are among the smallest pathogenic agents that humans have encountered, but while viruses may seem to be as little as it gets, there is another agent of disease that is even smaller: the viroid . Viroids are very simple, and very small, being made up of a single piece of circular RNA. Despite their small size, viroids still pose a major threat to several species of plants, and are responsible for such diseases as potato spindle tuber disease and cadang-cadang (a disease that infects coconut palms). Despite only being identified about half a century ago, these simple pathogens are likely to have been around for a lot longer than 50 years.

How long have they been around?

Viroids may be new to you, but research shows that they are actually likely to be relics of the past, with some scientists suggesting that viroids may have been some of the earliest agents of disease. Researchers cite the idea that life originally was RNA based, not DNA based, when hypothesizing about the origin of viroids, making the connection between the structure of viroids and the structure of early biological macromolecules. The simplicity of viroids leads scientists to believe that they could be a window into the past of life on Earth, as they are likely very similar to some of the earliest organic macromolecules that appeared on our planet.

If they are so simple, how do they replicate?

Once viroids infect a host, they use the host cell’s RNA polymerases to replicate themselves. From there the newly synthesized RNAs spread throughout the host organism, repeating the cycle of reproduction in various host cells. The simplicity of viroids makes reproduction very simple, as all it takes is the replication of a single short RNA strand. This can make viroid outbreaks very hard to contain.

How do viroids affect humans?

While viroids themselves cannot infect humans, they can infect many of the plants that humans rely on for foods such as potatoes and coconuts. Once a plant is infected by a viroid, the plant often has developmental issues and has its growth stunted. In this way viroids indirectly affect human well-being. Viroid outbreaks are particularly inconvenient for farmers because the only current way to stop an outbreak is to destroy all infected plants. This can severely limit sizes of harvests for farmers, which can be a major economic burden.


Viroids are especially applicable to our current unit of study in class, as we have been learning about the origin of life on Earth. Viroids are seen by many scientists as likely to be relics of the “RNA world” that saw the beginnings of life. It is very interesting to me personally that single circular strands of RNA have the ability to be agents of disease. Seeing these very simple biological molecules and comparing them to the complex forms of life that exist in the present helps me to appreciate the study of biology as a whole.


About Mr. Mohn

Biology Teacher

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