A virus is an infectious agent of small size and simple composition that can multiply only in living cells of animals, plants, or bacteria.

A virus particle is made up of genetic material housed inside a protein shell, or capsid. The genetic material, or genome, of a virus may consist o...

Most viruses vary in diameter from 20 nanometres (nm; 0.0000008 inch) to 250–400 nm. The largest viruses measure about 500 nm in diameter and are a...

Shapes of viruses are predominantly of two kinds: rods (or filaments), so called because of the linear array of the nucleic acid and the protein su...

When some disease-causing viruses enter host cells, they start making new copies of themselves very quickly, often outpacing the immune system’s pr...