An infectious particle that can reproduce only inside a living cell is called a virus. Viruses are extremely small infectious agents made of genetic material—DNA or RNA—enclosed in a protein coat, and some also have a lipid envelope.
Unlike bacteria, viruses do not have the cellular machinery needed to make energy, build proteins, or reproduce independently. They attach to a susceptible host cell and deliver their genetic material inside it. The infected cell is then redirected to produce viral components, which assemble into new virus particles.
Viruses infect organisms across every major branch of life, including humans, animals, plants, fungi, and bacteria. Human diseases caused by viruses include influenza, measles, COVID-19, and HIV infection. Antibiotics work against bacteria, not viruses, which is why treatment and prevention strategies differ.
The word “virus” is often used for both a complete infectious particle and the broader biological entity. A complete particle outside a host cell is sometimes called a virion.