A virus is a microscopic infectious agent that infects and replicates only inside living cells of a host organism. Viruses can be found in all ecosystems and are the most abundant type of biological entity on Earth. They are the smallest and most diverse of all living things.
Viruses have a core of genetic material, either DNA or RNA, covered by a shell or capsid made from proteins. The shapes of capsids vary, from icosahedral to helically complex. The capsid may be surrounded by a spiky covering called an envelope, which helps viruses bind to and enter host cells. Some viruses have additional surface proteins that recognize and bind to specific receptors on the surfaces of host cells. Once a virus attaches to the cell, it hijacks the host cell’s normal processes to begin replicating its own genetic material and producing more viruses. Eventually, the host cell will explode (lyse) or run out of energy and shut down its normal functions.
Because viruses cannot survive and reproduce on their own, they must infect living cells to thrive and spread. Viruses can be transferred from one person to another by touching an object or surface that has been touched by someone who is carrying the virus, such as countertops, doorknobs, or cell phones. Viruses can also be transmitted through vaginal, oral, or anal sex, or by getting a bite from an infected mosquito or tick. Viruses can remain active on surfaces for some time, making them contagious to anyone who touches the surface. Viruses can also infect people through airborne droplets, such as sneezes or coughs.
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