Scientists Say: Virus

These non-living particles infect living cells

a red spherical object with many thin knobs poking out of it floats against a black background

Spike proteins cover the surface of the virus that causes COVID-19 (illustrated).

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Virus, (noun, “VY-rus”)

A virus is a non-living, microscopic germ that infects living things. Viruses cause infections by inserting their own genetic material into living cells. This causes the cell to follow the instructions spelled out in the virus’ DNA or RNA, instead of its own. As a result, the cell creates more viruses.

Viruses infect every form of life on Earth. Even bacteria suffer from viral infections. Examples of viral diseases in humans include measles, the flu, COVID-19 and the common cold.

All viruses contain genetic material — either DNA or RNA — inside a protein shell called a capsid. But viruses can also have various other gadgets and accessories that help them infect cells. Some have iron-tipped spikes for punching holes in bacterial cell walls. “Enveloped” viruses get their name from an outer layer similar to a cell membrane that encapsulates the cell. The SARS-CoV-2 virus, which causes COVID-19, is one example.

Viruses also come in all different shapes. A bacteria-infecting virus called a bacteriophage looks like a six-legged insect. The tobacco mosaic virus, which infects plants, appears tube-like.

Some scientists say viruses are not living, because these germs cannot reproduce on their own. But viruses do have some of the traits of living things. For example, virus species can change over time, or evolve. This can happen if there is a spontaneous mutation, or change, in the virus’ DNA or RNA. This also can happen when two viruses infect the same cell. When two viruses share the same space, they can swap pieces of their DNA or RNA. That mixing can result in a virus with new properties — such as resistance to medicines or vaccines.

This is why you need a flu shot every year. Over time, flu viruses swap DNA, evolving into different strains. And last year’s flu shot might not protect against the strain making the rounds this year.

In a sentence

The HearNPV virus changes its caterpillar host’s behavior, making caterpillars more likely to be consumed by predators.

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Katie Grace Carpenter is a science writer and curriculum developer, with degrees in biology and biogeochemistry. She also writes science fiction and creates science videos. Katie lives in the U.S. but also spends time in Sweden with her husband, who’s a chef.

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