Viruses are nonliving structures with a genome that infect cells and use their cellular machinery to replicate.
Although not technically living organisms, viruses have some lifelike qualities. Viruses have their own DNA (or RNA), with a protein covering, or capsid, enclosing the viral genetic material. They are extremely small, even when compared with bacteria. Viruses are able to replicate, but only by taking advantage of a host cell's enzymes and ribosomes to make proteins and replicate their genome. They cannot reproduce on their own, which is the primary reason that they are not considered alive. They are able to change through mutation of the genome. No group of organisms is safe from viruses. There are viruses that target bacteria (called bacteriophages or just phages), plants, and individuals of all other kingdoms (second largest taxonomic grouping). Viral shapes, such as spherical or icosahedral, vary considerably based on the virus's host type. However, all viruses infect their hosts in a similar way. When a virus enters a cell, it falls apart into its two components: nucleic acids and proteins. The virus then uses the cell's machinery to copy its parts and construct new viral particles. When a virus enters a human body, an immune response is launched and antibodies are formed to attack the virus. If the virus ever returns to the same body, the immune system is prepared, and it launches an attack faster and more efficiently than the first time. Vaccines are a way to take advantage of this immune response. Weakened or dead viral particles can trick the immune system into creating antibodies, proteins that help eliminate pathogens from the body. If the live virus is ever introduced to the body at a later time, the body is prepared to fight it off. A vaccinated person may never even know that they were exposed to the virus.