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A virus is basically a tiny bundle of genetic material—either DNA or RNA—carried in a shell called the viral coat, or capsid, which is made up of bits of protein called capsomeres. Some viruses have an additional layer around this coat called an envelope. That's basically all there is to viruses.
The polio virus once crippled millions.
The T4 bacteriophage is a virus that invades bacterial cells.
Gold clusters bound to the knob protein of Adenovirus.
Microbes are single-celled organisms that can perform the basic functions of life — metabolism, reproduction, and adaptation.
Except viruses. Viruses can’t metabolize nutrients, produce and excrete wastes, move around on their own, or even reproduce unless they are inside another organism’s cells.
They aren’t even cells.
Yet viruses have played key roles in shaping the history of life on our planet by shuffling and redistributing genes in and among organisms and by causing diseases in animals and plants. Viruses have been the culprits in many human diseases, including smallpox, flu, AIDS, certain types of cancer, and the ever-present common cold.