A virus is a microscopic infectious agent that can replicate only inside the living cells of an organism. Viruses are made up of genetic material (either DNA or RNA) encased in a protein coat, and sometimes they have an outer lipid envelope. Unlike cellular life forms, viruses do not possess the cellular machinery necessary for metabolism and reproduction, and therefore they must hijack the host cell's machinery to reproduce and propagate.
Viruses can infect a wide variety of hosts, including animals, plants, fungi, and bacteria (the latter are infected by viruses known as bacteriophages). The interaction between a virus and its host can lead to a range of outcomes, from benign to devastating, depending on factors such as the type of virus, the host's immune response, and the virulence of the strain.
Viruses are responsible for a multitude of diseases in humans, animals, and plants, including the common cold, influenza, HIV/AIDS, and COVID-19. Vaccines and antiviral medications are some of the methods used to prevent or treat viral infections.