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A computer virus is malware that reproduces by inserting copies of itself into a host program or data file. The key distinction is that a virus needs that host to spread; “malware” is the broader term for malicious software, including threats that spread in other ways. Knowing the difference makes it easier to understand how infections happen and which everyday precautions help reduce risk.
What makes a computer virus different from other malware?
A virus copies itself into a host program or data file, so its replication is tied to that host. NIST describes viruses as self-replicating through insertion into host programs or data files in Special Publication 800-83 Revision 1.
Malware is the wider category of software designed to compromise data, applications, or an operating system. It includes viruses as well as worms, Trojan horses, spyware, and some forms of adware. A worm can spread independently, unlike a virus, which relies on a host program or file. Calling every malicious program a “virus” blurs this useful difference; see the NIST glossary for related terminology.
What are the main types of computer viruses?
NIST’s 2013 guide describes three basic forms based on what a virus infects. These are technical categories, not a ranking of current prevalence or a complete inventory of every virus variant.
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| Type | What it infects | How it replicates |
|---|---|---|
| File-infecting | Executable programs | Attaches copies of itself to executable files, which can spread when an infected program is run. |
| Boot-sector | A drive’s boot record or the boot sector of removable media | Targets the information used to start a computer or access removable media. |
| Multipartite | Executable files and boot sectors | Combines file-infecting and boot-sector traits. |
The classifications are from NIST SP 800-83 Revision 1. They help explain what a virus targets; they do not establish which forms are most common today.
How can a computer get a virus?
Infection may begin with an action such as opening an attachment or running a download, but not every route depends on that. A compromised webpage, for example, may exploit a vulnerability in outdated software. Microsoft outlines several common exposure routes in its guidance on protecting a PC from viruses.
- Email attachments and links: Unexpected messages may disguise malicious files as receipts, invoices, or other plausible documents. Attachment-based infection typically requires opening or running the file.
- Bundled downloads: Unwanted software can be packaged with a program, particularly when it comes from a third-party download source. Installation prompts can reveal optional components.
- Compromised or malicious webpages: A vulnerable browser or other software may be exploited when visiting a site; a user does not always have to download a file manually.
- Removable drives: An infected USB drive or other removable media can expose a computer when connected or when files on it are used.
How can you reduce the risk of infection?
No single measure guarantees that a computer will stay free of malware. A layered routine reduces common exposures:
- Be cautious with unexpected attachments and links, even when a message looks routine. Verify the sender and the reason for the file through a separate, trusted channel if unsure.
- Download software from its official vendor or another source you trust. Read installation screens rather than accepting bundled options automatically.
- Keep the operating system, browser, and other software updated so known vulnerabilities are less likely to remain exposed.
- Avoid connecting unknown USB drives or opening files from unfamiliar removable media.
- Use current real-time security protection. NIST defines antivirus software as software that monitors a computer or network to identify malware and help prevent or contain incidents; that makes it a useful layer, not a guarantee.
For Windows users, Microsoft says Microsoft Defender Antivirus is built into modern Windows versions. The details on Microsoft’s virus-protection page include older Windows applicability labels, so check the guidance for the Windows version in use. For other operating systems, use the security protections and update guidance that apply to that platform. The NIST definition of antivirus software is in its glossary.
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Do slowdowns or pop-ups prove that a computer has a virus?
No single symptom proves an infection. Sluggish performance, unexpected pop-ups, or other unusual behavior can have causes unrelated to malware. Treat a concerning change as a reason to check your security protection and the device’s condition, rather than as a diagnosis on its own.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What should you do if you suspect an infection?
If you suspect a serious incident, disconnecting the device from networks may help limit further exposure while you seek appropriate support. The steps that safely identify and remove malware depend on the device and situation; the general guidance here is not a complete cleanup procedure. Avoid assuming that one symptom confirms infection or that one generic fix will resolve every case.
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