A virus is an infectious particle that reproduces by "commandeering" a host cell and using its machinery to make more viruses.
A virus is made up of a DNA or RNA genome inside a protein shell called a capsid. Some viruses have an internal or external membrane envelope.
Viruses are very diverse. They come in different shapes and structures, have different kinds of genomes, and infect different hosts.
Viruses reproduce by infecting their host cells and reprogramming them to become virus-making "factories."