A virus is a submicroscopic infectious agent that only replicates inside the cells of an organism.
Viruses affect all cellular life, so both eukaryotic cells and prokaryotic cells.
When infected, a host cell is often forced to rapidly produce many copies of the original virus, and that is the sole way a virus can reproduce.
Many viruses studied are spherical and have a diameter between 20 and 300 nanometers.
Viruses display a wide diversity of sizes and shapes.