A computer virus is malware that copies itself by inserting its code into another program or file. It needs that infected host to run before it can activate. A worm or Trojan is also malware, but neither is a virus under this host-based definition: worms spread between devices, while Trojans trick people into installing them.
What is a computer virus?
NIST’s glossary defines a virus in terms of how it replicates: it modifies other programs or files to include a copy of itself. The infected program or file is the virus’s host; running that host activates the virus. In other words, a virus does not run independently of its host.
A virus may carry a harmful payload, but damage is not what makes it a virus. The defining trait is host-based replication. A virus might act when its infected host runs, or it might wait for a condition such as a particular date. Those are possible behaviors, not things every virus does. NIST’s glossary entry for “virus” draws on definitions including RFC 4949 as adapted in NIST SP 800-82r3.
How a virus differs from a worm and a Trojan
These terms describe different malware behaviors. Calling every malicious program a “virus” blurs the distinction between how it spreads and how it reaches a victim.
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| Type | How it spreads or gets installed | Does it need a host program? |
|---|---|---|
| Virus | Copies itself into another program or file; that host must run to activate it. | Yes. It infects a host. |
| Worm | Propagates from device to device, for example through email, messaging, file sharing, network shares, or removable drives. Some worms exploit software vulnerabilities. | No infected host file is required for the defining spread between devices. |
| Trojan | Poses as a harmless or legitimate item to persuade someone to install it; it does not spread by itself. | No. Its defining feature is deception that leads to installation, not host-based replication. |
These distinctions follow Microsoft’s malware classification criteria and NIST’s virus definition. A malicious download might be a Trojan, a virus, another kind of malware, or a combination of behaviors; the delivery route alone does not identify it.
Common virus types and examples
File or program infectors
A file or program infector inserts its code into another file or program. When someone runs the infected host, the virus can activate and may attempt to infect additional files. The host dependency—not a particular payload—is what makes this a virus.
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Macro viruses
A macro virus spreads through infected documents, such as Word or Excel files. Opening an infected document can run its malicious macro code. Microsoft describes macro viruses as a malware category that spreads through documents; the document is the relevant host in this example. Microsoft’s classification guidance provides this distinction.
Worms and Trojans are related malware, not virus examples
Worms are worth knowing about because they can spread through channels such as email, messaging, file sharing, network shares, or removable drives, and some exploit vulnerabilities. Trojans use a different route: they appear harmless or legitimate to persuade a person to install them. Neither should be called a virus solely because it is malicious.
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How malware reaches computers
Malware, including some viruses, can arrive through deceptive email attachments or webpages that exploit software vulnerabilities. These are general malware delivery routes; they do not mean that every attachment or compromised webpage delivers a virus. The infection route by itself does not tell you whether the malware is a virus, worm, Trojan, or another type. Microsoft’s malware infection guidance describes these routes.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How to reduce the risk of infection
- Treat unexpected attachments cautiously. Verify an attachment through a separate, trusted channel before opening it, especially if the message creates urgency or asks you to enable document content.
- Download software from official vendor sources. Avoid installers from unfamiliar download pages, and read each installation prompt so you can spot optional software or changes you did not request.
- Keep browsers and other software updated. Updates can address vulnerabilities that malicious webpages may exploit. Apply updates through the software’s normal update mechanism or the vendor’s official site.
- Use the protection built into your operating system. Modern Windows versions include Microsoft Defender Antivirus. Windows 11 documentation describes Defender Antivirus as monitoring for malware and receiving automatic updates; this Windows-specific protection is a useful layer, not a guarantee that a device cannot be infected.
Microsoft’s consumer guidance covers infection routes and safer computing habits, while its protection guidance discusses built-in Defender Antivirus. For Windows 11 details, see Microsoft’s virus and threat protection documentation.
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