Virusoid is a term used to describe a sub-viral particle. It is a small RNA molecule that lacks some of the key elements necessary for replication. There are other terms that can be used to refer to virusoid. These include virus-like particles, viroids, satellite viruses, and sub-viral agents. These particles are considered as intermediate between viruses and other forms of replicating molecules such as plasmids and transposons. They are not harmful to humans, but can sometimes affect plants and animals. Scientists continue to study these particles to understand their structure and functions in order to develop better ways of combating diseases caused by them.