Final answer:
A virion is the term used for an individual virus particle, consisting of a nucleic acid core surrounded by a protein coat or capsid, often with additional structures like an envelope. Virions are acellular and require a host cell to reproduce. They are microscopic, with most being invisible to regular light microscopes.
Step-by-step explanation:
An individual virus particle is called a virion. This structure is the infectious form of a virus when it is outside of a host cell, composed of a nucleic acid core, an outer protein coating, or capsid, and sometimes an additional outer envelope. A virion, much smaller than prokaryotic and eukaryotic cells, is acellular and lacks cellular components like organelles and ribosomes. This absence of cellular structures means viruses cannot reproduce or make proteins without a host.
Despite not being classified as living due to their simplicity and reliance on a host cell for replication, viruses do have genetic material and can evolve. Viral particles can range in size from 20 nm to 900 nm. The nanoscale size of most viruses makes them invisible under regular light microscopes, and only some large viruses, like those from the poxvirus family, are exceptions.
During the infection process, viruses often cause damage to host cells through cytopathic effects, which can be visualized when culturing viruses in a laboratory setting. These effects can result in the formation of multinucleated giant cells from the fusion of virus-infected and neighboring uninfected cells, often seen in certain viral infections like cytomegalovirus (CMV).