virus

noun

vi·​rus ˈvī-rəs 

plural viruses

1

a

: any of a large group of submicroscopic infectious agents that are usually regarded as nonliving extremely complex molecules, that typically contain a protein coat surrounding an RNA or DNA core of genetic material but no semipermeable membrane, that are capable of growth and multiplication only in living cells, and that cause various important diseases in humans, animals, and plants

also : filterable virus

b

: a disease or illness caused by a virus

c

: the causative agent of an infectious disease

2

: something that poisons the mind or soul

… the force of this virus of prejudice …

—V. S. Waters

3

: a computer program that is usually disguised as an innocuous program or file, that often produces copies of itself and inserts them into other programs, and that when run usually performs a malicious action (such as destroying data or damaging software)

4

archaic : venom sense 1

About Viruses and Bacteria: Usage Guide

Viruses differ from bacteria in several important ways. Viruses are not living organisms; they can only replicate in the cells of a host. By contrast, bacteria are single-celled organisms that can reproduce on their own and are many times larger than viruses. While both viruses and bacteria can cause disease, most bacteria are harmless and many are beneficial.

Examples of virus in a Sentence

Is the illness caused by bacteria or a virus?

I think I have the virus that's going around this winter.

The software checks your hard drive for viruses.

Recent Examples on the Web

Amid the second surge of the virus, in August 2020, the first ‘covid novel’ arrived—Ali Smith’s Summer, the last of a seasonal quartet, written during Britain’s early lockdown, which includes real-time descriptions of the pandemic’s onset. —Literary Hub, 6 Nov. 2025

But only about 23% of American adults got a covid shot during the 2024-25 virus season, well below the 47% of American adults who got a flu shot. —Phillip Reese, CNN Money, 5 Nov. 2025

Lupus is an autoimmune disease in which the body's immune system attacks healthy tissues, rather than targeting the bad bacteria and viruses, causing inflammation that can affect the joints, skin, kidneys, blood cells, brain, heart, and lungs. —Madison E. Goldberg, PEOPLE, 4 Nov. 2025

Researchers also fed ferrets, animals known to be highly vulnerable to the H5N1 virus, both contaminated raw milk and raw milk cheese. —Pranjal Malewar, New Atlas, 3 Nov. 2025

Word History

Etymology

Middle English, "pus, discharge from a sore, semen," borrowed from Latin vīrus (neuter) "venom, poisonous fluid, acrid element in a substance, secretion with medical or magical properties," going back to an Indo-European base *u̯is-/*u̯īs- "poison, venom," whence also Middle Irish fí "venom, poison, evil," Greek īós "poison," Tocharian A wäs and Tocharian B wase, Sanskrit viṣáṃ, Avestan viš, viša- (also vīš?); (sense 1) borrowed from German, borrowed from Latin

Note: The application of Latin vīrus to the submicroscopic infectious agents now considered viruses (rather than to any infectious agent) was apparently first made by the Dutch microbiologist Martinus Beijerinck (1851-1931) in "Ueber ein Contagium vivum fluidum als Ursache der Fleckenkrankheit der Tabaksblätter," Verhandelingen der Koninklijke Akademie van Wetenschappen te Amsterdam, Tweede Sectie, Deel VI, no. 5 (1898). Beijerinck, in studying tobacco mosaic virus, mistakenly believed that the agent was a fluid (contagium vivum fluidum, "living fluid infection") because it passed through filters capable of trapping bacteria. — The neuter gender of vīrus suggests that it was originally an s-stem; forms in text other than the nominative and accusative are perhaps found only in Lucretius. The length of the vowel in Latin, Irish, and Greek, in contrast to the short vowel in Tocharian and Indo-Iranian, has been variously accounted for. M. Mayrhofer (Etymologisches Wörterbuch des Altindoarischen) suggests that the etymon was originally a root noun, *u̯īs, *u̯is-ó-, with lengthening of the monosyllabic vowel; the daughter languages then generalized one or the other form.

First Known Use

1599, in the meaning defined at sense 4

Phrases Containing virus

• AIDS virus
• coxsackie virus
• delta virus
• DNA virus
• Ebola virus
• EB virus
• Epstein-Barr virus
• filterable virus
• hepatitis delta virus
• hepatitis D virus
• human immunodeficiency virus
• Marburg virus
• Norwalk virus
• parainfluenza virus
• polio virus
• Powassan virus
• Powassan virus disease
• respiratory syncytial virus
• RNA virus
• simian immunodeficiency virus
• slow virus
• tobacco mosaic virus
• vaccinia virus
• West Nile virus
• Zika virus

Rhymes for virus

• gyrus
• iris
• pyrus
• adenovirus
• baculovirus
• coronavirus
• enterovirus
• poliovirus
• antivirus
• herpesvirus
• parvovirus
• reovirus

Kids Definition

virus

noun

vi·​rus ˈvī-rəs 

plural viruses

1

: any of a large group of very tiny infectious agents that are too small to be seen with the ordinary light microscope but can often be seen with the electron microscope, that are usually regarded as nonliving complex molecules, that have an outside coat of protein around a core of RNA or DNA, that can grow and multiply only in living cells, and that cause important diseases in plants and animals including human beings compare filterable virus

2

: a disease or illness caused by a virus

3

: a computer program that is usually hidden within another seemingly harmless program and that produces copies of itself and inserts them into other programs and usually performs a malicious action (as destroying data) compare trojan horse sense 2, worm sense 5

Etymology

from Latin virus "poison, venom, secretion"

Medical Definition

virus

noun

vi·​rus ˈvī-rəs 

1

a

: the causative agent of an infectious disease

b

: any of a large group of submicroscopic, infectious agents that are usually regarded as nonliving, extremely complex molecules or sometimes as very simple microorganisms, that typically contain a protein coat surrounding an RNA or DNA core of genetic material but no semipermeable membrane, that are capable of growth and multiplication only in living cells, and that cause various important diseases in humans, animals, and plants

also : filterable virus

c

: a disease caused by a virus

2

: an antigenic but not infectious material (as vaccine lymph) obtainable from a case of an infectious disease