Definition

A virus is a tiny particle, much smaller than a bacterium, that can only make copies of itself by getting inside a living cell and hijacking its machinery, which is how colds, the flu, and chickenpox spread from cell to cell and person to person; outside a cell a virus is not really alive, just sitting inert like a message with no one to read it. More precisely, a virus is a non-cellular infectious agent made of genetic material, DNA or RNA, enclosed in a protein capsid, sometimes with an outer lipid envelope, unable to carry out metabolism or reproduce without a host cell's ribosomes and enzymes; antibiotics do not kill viruses, but vaccines can prevent many viral diseases and some antiviral drugs slow viral replication. At the molecular level, viruses are classified by genome type, capsid symmetry, and the presence of an envelope, and infection proceeds through attachment to a specific receptor, entry, uncoating, genome replication and gene expression using host machinery, assembly, and release by lysis or budding; RNA viruses mutate rapidly because their polymerases lack proofreading, retroviruses such as HIV reverse-transcribe RNA into DNA that integrates into the host genome, and bacteriophages, viruses that infect bacteria, are the most abundant biological entities on Earth.