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
V. S. Waters3
: 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
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
The virus is typically spread through deep bite wounds, according to the ASPCA.
—John W. Dean, MSNBC Newsweek, 7 Apr. 2025
There was an outbreak of a related hantavirus, Seoul virus, among people who owned pet Norway rats.
—Judy Stone, Forbes.com, 7 Apr. 2025
The measles virus can remain contagious in the air or on contaminated surfaces for up to two hours.
—USA Today, 7 Apr. 2025
On the left are children under 5, whose still-maturing immune systems might struggle to fend off the virus.
—Sarah Zhang, The Atlantic, 7 Apr. 2025
See All Example Sentences for virus
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John W. Dean, MSNBC Newsweek, 7 Apr. 2025
There was an outbreak of a related hantavirus, Seoul virus, among people who owned pet Norway rats.
—Judy Stone, Forbes.com, 7 Apr. 2025
The measles virus can remain contagious in the air or on contaminated surfaces for up to two hours.
—USA Today, 7 Apr. 2025
On the left are children under 5, whose still-maturing immune systems might struggle to fend off the virus.
—Sarah Zhang, The Atlantic, 7 Apr. 2025
See All Example Sentences for virus
Cite this Entry
“Virus.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/virus. Accessed 14 Apr. 2025.
Copy CitationKids 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
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