Small particle that does not exhibit signs of life, but can reproduce itself within a living cell. A virus particle is called a virion; it consists of a nucleic acid (DNA or RNA) core and a protein coat, called a capsid. A virus reproduces by infecting a host cell and taking over the nucleic acid of that host cell, making more virus nucleic acid and protein. As new virus particles develop, the host cell bursts, releasing the new virus particles. Viruses are responsible for many human diseases.