virus

American  

[vahy-ruhs] / ˈvaɪ rəs /

noun

• viruses
  plural

1. an ultramicroscopic (20 to 300 nanometers in diameter), metabolically inert, infectious agent that replicates only within the cells of living hosts, mainly bacteria, plants, and animals: composed of an RNA or DNA core, a protein coat, and, in more complex types, a surrounding envelope.

2. a viral disease.

   He stayed home sick with a virus for nearly two weeks.

3. a corrupting influence on morals or the intellect; poison.

4. a segment of self-replicating code planted illegally in a computer program, often to damage or shut down a system or network.

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virus British  

/ ˈvaɪrəs /

noun

1. any of a group of submicroscopic entities consisting of a single nucleic acid chain surrounded by a protein coat and capable of replication only within the cells of living organisms: many are pathogenic

2. informal a disease caused by a virus

3. any corrupting or infecting influence

4. computing an unauthorized program that inserts itself into a computer system and then propagates itself to other computers via networks or disks; when activated it interferes with the operation of the computer

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virus Scientific  

/ vī′rəs /

• viruses
  plural

1. Any of various extremely small, often disease-causing agents consisting of a particle (the virion), containing a segment of RNA or DNA within a protein coat known as a capsid. Viruses are not technically considered living organisms because they are devoid of biological processes (such as metabolism and respiration) and cannot reproduce on their own but require a living cell (of a plant, animal, or bacterium) to make more viruses. Viruses reproduce first either by injecting their genetic material into the host cell or by fully entering the cell and shedding their protein coat. The genetic material may then be incorporated into the cell's own genome or remain in the cytoplasm. Eventually the viral genes instruct the cell to produce new viruses, which often cause the cell to die upon their exit. Rather than being primordial forms of life, viruses probably evolved from rogue pieces of cellular nucleic acids. The common cold, influenza, chickenpox, smallpox, measles, mumps, yellow fever, hemorrhagic fevers, and some cancers are among the diseases caused by viruses.

2. Computer Science A computer program that duplicates itself in a manner that is harmful to normal computer use. Most viruses work by attaching themselves to another program. The amount of damage varies; viruses may erase all data or do nothing but reproduce themselves.

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virus ¹ Cultural  

1. See computer virus.

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virus ² Cultural  

1. plur. viruses A minute organism that consists of a core of nucleic acid surrounded by protein. Viruses, which are so small that a special kind of microscope is needed to view them, can grow and reproduce only inside living cells. (See under “Life Sciences.”)

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virus ³ Cultural  

1. plur. viruses Microorganisms consisting of DNA and RNA molecules wrapped in a protective coating of proteins. Viruses are the most primitive form of life. They depend on other living cells for their reproduction and growth. (See under “Medicine and Health.”)

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Usage

What is a virus? A virus is an ultramicroscopic agent that can only reproduce in living cells and is spread through infection. Many viruses cause diseases as part of their reproduction process. Viruses are only 20 to 300 nanometers—so small that even microscopes can’t see them. Viruses are also very simple. They consist of a core made of DNA or RNA, a protein coat that surrounds the core, and sometimes an envelope that surrounds the core. A virus can’t reproduce on its own. Instead, it will infect a living cell and force it to make more copies of the virus. When the virus does this, it stops the cell from whatever it was doing before and, eventually, kills the cell. Viruses are infectious, meaning they often cause symptoms that allow fluids with copies of the virus to spread to other organisms. For example, if you have the flu and cough on another person, your virus-containing saliva and mucus will enter the other person’s body and allow the virus to infect their cells. Because viruses remain inside living cells, it is often impossible to kill the virus without also killing the cell. Usually, your immune system is the only thing that can safely fight a virus.

Discover More

Viruses cause many diseases. (See viral infection.)

Other Word Forms

Derived Forms

• antivirus
  adjective
• virus-like
  adjective
• viruslike
  adjective

Inflected Forms

Nouns

• viruses
  plural

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Etymology

Origin of virus

First recorded in 1590–1600; from Latin vīrus “slime, poison”; akin to Sanskrit viṣá-, Avestan viša-, Greek iós (from earlier wiós, from still earlier wisós ), all meaning “poison, venom”

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Explanation

Whether in the human body or on a computer, a virus is a disease-causing agent. If you come down with a nasty stomach virus, be sure to drink lots of water, get lots of rest, and stay near the toilet. Virus comes from a Latin word that was spelled the same way. It specifically referred to the poisonous sap of certain plants, which explains why it sounds similar to viscous, a word meaning "sticky." Nowadays, it’s used to describe something that causes infectious diseases, specifically a tiny, self-replicating agent made up of genetic material and protein.

Example Sentences

See Examples For:

virus viruses

Experts have also ruled out West Nile virus and heavy metals as factors that could have contributed to her death.

From Los Angeles Times ● Oct. 15, 2026

Detecting his illness early had improved his chances of recovery and limited the chance of him passing the virus on to others, the ministry said, as it urged the public to "stay calm but vigilant".

From BBC ● Oct. 14, 2026

RSV is a common virus that circulates every winter.

From BBC ● Oct. 13, 2026

The California Department of Public Health reports that West Nile virus activity in the state, which is transmitted from infected mosquitoes, has reached a five-year high, with over 250 cases to date and 19 fatalities.

From The Wall Street Journal ● Oct. 9, 2026

Through this encounter with the fever their blood automatically produced antibodies that either fought off the yellow fever virus entirely or reduced its impact on the individual significantly.

From "An American Plague: The True and Terrifying Story of the Yellow Fever Epidemic of 1793" by Jim Murphy

And certainly social media, now overrun by bots, increasingly feels as if every conversation is between different viruses.

From Salon ● Oct. 12, 2026

"So we have really high rates of sickness and burnout among staff - and this is before the viruses really hit."

From BBC ● Oct. 11, 2026

More patients, including children, have been arriving at A&E departments with flu and other respiratory viruses.

From BBC ● Oct. 8, 2026

Human clinical trials of their antibody drugs against bird flu and other viruses in the flu family are planned for next year.

From The Wall Street Journal ● Sep. 28, 2026

They represent thousands of different species; there are tiny plants, tiny fungi, tiny viruses, and tiny animals.

From "Phineas Gage" by John Fleischman

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Related Words

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• pathogen
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