Viroids are infectious elements capable of causing disease in their hosts. Viroids can only make plants sick, since there is still no knowledge that one has made a human or other animal sick. Like viruses, viroids are not considered living beings, since they do not have any type of metabolic activity.
Theodor Otto Diener is the plant expert who discovered the first viroid when analyzing the cause of the disease of the potato spindle tuber, which at first was thought to be due to a virus, but in reality it was a viroid.
Regarding their characteristics, viroids have little structural and genetic complexity, rather they are considered as an extremely intense form of parasitism. It is only made up of single-stranded, short-length RNA particles. They can come in the form of circles or rods. They do not have any type of RNA activity and in order to replicate, they need the cells that they contaminate. Given their location, it is believed that they cause the disease by hindering the gene regulation of the host cell in the modification phase of messenger RNA.
It is currently known that at least 300 species of viroids can infect only higher plants, whether they are woody or herbaceous. The host variety of viroids is very extensive. The most common diseases caused by viroids are: apple skin affected by bruises, tomato atrophy, the disease of the potato or potato filiform tuber, the disease of roasted avocado, etc.
Contrary to plant viruses, viroids can replicate, accumulate, and display symptoms more effectively at high temperatures and at an equally high magnitude of light.
