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A computer virus is malicious, self-replicating code that inserts copies of itself into host programs or data files. It commonly activates when someone opens an infected file or runs an infected program. “Virus” is often used casually for any malicious software, but viruses are one category within the broader malware family.
The safest approach combines updated software, cautious online behavior, limited account privileges, security scanning, and backups that can be restored if an incident occurs.
What is a computer virus?
NIST defines a virus as code that “self-replicates by inserting copies of itself into host programs or data files.” Unlike a standalone malicious program, a traditional virus depends on a host. User interaction—such as opening a file, launching a program, or booting from infected media—can trigger execution and further replication.
That definition matters because not every unwanted program is a virus. Malware is the umbrella term for software or code intended to damage systems, disrupt operations, steal information, spy on users, or gain unauthorized access.
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Virus types in the classic NIST taxonomy
NIST’s categories are a useful technical foundation, not a complete catalogue of every modern threat.
| Type | What it infects | Typical behavior |
|---|---|---|
| Compiled file infector | Executable programs | Attaches code to an executable so the virus can run when the host program is launched. |
| Boot-sector virus | Boot records on disks or removable media | Runs during startup and can interfere with the system before the operating system fully loads. |
| Multipartite virus | Both executable files and boot sectors | Combines file-infection and boot-sector behaviors, allowing it to persist through more than one route. |
Modern attacks may combine multiple techniques or use delivery methods that do not fit neatly into these older labels. The practical question is what the code does, how it spreads, what access it gains, and how it can be removed.
How viruses differ from other malware
Distinguishing malware types helps you understand likely exposure and the right response. A single incident can involve more than one category.
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| Malware category | Defining characteristic | Possible harm |
|---|---|---|
| Virus | Replicates by inserting copies into host programs or data files. | Corrupted or altered files, crashes, disrupted programs, or further spread through shared files and media. |
| Worm | Self-contained code that can spread without attaching to an existing host file. | Rapid network or device spread, congestion, service disruption, and unauthorized activity. |
| Trojan horse | Presented as legitimate or useful software while performing a malicious function. | Unauthorized access, additional malware installation, or theft of information. |
| Spyware | Monitors activity or collects information without appropriate consent. | Exposed browsing activity, credentials, personal data, or other sensitive information. |
| Ransomware | Restricts access to files or devices, commonly to demand payment. | Unavailable data, operational downtime, and possible permanent loss if no usable backup exists. |
Older NIST guidance also discusses malicious mobile code, blended attacks, backdoors, rootkits, and keystroke loggers. These terms describe additional techniques or tools rather than replacements for the basic virus definition.
What damage can a virus cause?
Impact depends on the specific malware, the permissions of the compromised account, the files it reaches, and how quickly it is contained. Possible consequences include:
- Unauthorized reading, editing, deletion, or theft of stored data.
- Compromised passwords, session tokens, or other credentials.
- Unexpected monitoring of activity or collection of personal information.
- Program failures, system instability, slowdowns, or crashes.
- Spread to shared drives, removable media, email contacts, or other devices.
- Loss of access to files or devices when ransomware is involved.
A virus does not automatically cause every item on this list. Treat the behavior and capabilities of the particular infection as decisive, rather than assuming the most severe outcome from the word “virus” alone.
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Signs that a computer may be infected
The Federal Trade Commission lists several warning signs:
- Unusually slow performance or rapidly draining battery.
- Unexpected errors, freezes, or crashes.
- Failure to shut down or restart normally.
- Persistent popups or browser redirects.
- A changed home page, search engine, toolbar, or unfamiliar browser icon.
- New programs or icons that you did not install.
These clues are not a diagnosis. A failing drive, unwanted browser extension, buggy update, low storage, or another software problem can look similar. One symptom alone is not proof of a virus.
How to protect your computer
Keep the operating system and applications patched
Enable automatic updates for the operating system, web browser, and security software when available. Patches close known weaknesses that attackers may otherwise exploit. Restart when an update requires it rather than postponing indefinitely.
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Use reputable security software
Install security software from a reliable provider, keep its automatic updates enabled, and use real-time protection where supported. Run an on-demand scan when you have a reason to investigate. Do not purchase security software because of an unexpected call, popup, or message claiming that someone remotely detected a problem; that is a common technical-support scam.
Handle links, attachments, and downloads carefully
- Do not open unexpected email attachments or links, even when the message appears urgent.
- Navigate to a service by typing its known address or using a saved bookmark instead of clicking a suspicious message.
- Close deceptive popups rather than downloading the “scanner” they advertise.
- Install software only from a source you trust, and avoid pirated or modified installers.
- Verify an attachment or payment request through a separate, known channel.
Limit everyday privileges
Use a standard (non-administrator) account for routine work. Keep administrator credentials for software installation and system changes. Limiting privileges can reduce what malicious code can alter if it runs.
Protect data and accounts
Use encryption where appropriate, strong unique passwords, and multifactor authentication for important accounts. Stay alert for phishing messages that request credentials, payment, or urgent action.
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Backups: your recovery safety net
Backups can make corrupted, deleted, or encrypted data recoverable, but a backup does not prevent infection and does not prove that a computer is clean.
CISA recommends copying important data to an external hard drive or a properly vetted cloud service. Choose a method that remains available if the computer is compromised, supports restoration when needed, and offers appropriate capacity and encryption. Keep at least one backup separated from the computer—such as disconnected storage or a separately protected cloud account—so malware cannot simply encrypt or delete every copy. Test that files can actually be restored.
What to do if you suspect an infection
- Stop sensitive activity. Do not enter passwords, banking details, or other confidential information on the suspected device.
- Disconnect when appropriate. Disconnect from the network if doing so will not disrupt an essential response or evidence-preservation process; this can limit communication and spread.
- Update protection. From a trusted connection when possible, update the security software already installed.
- Run a full scan. Follow the security software’s instructions and allow it to quarantine or remove detected threats.
- Check accounts from a clean device. Change passwords that may have been exposed, beginning with email and financial accounts, and enable multifactor authentication.
- Restore cautiously. If files are damaged or encrypted, use a known-good backup only after the device has been cleaned and patched.
- Get expert help if needed. Seek qualified technical assistance if scans cannot remove the problem, the device remains unstable, or business, financial, or sensitive personal data may be involved.
Keep records of suspicious messages, filenames, alerts, and timing. Do not pay an unsolicited “support” caller or give remote access based solely on a popup or phone message.
Quick Recap
Virus protection checklist
- Automatic operating-system, browser, and security updates are enabled.
- Security software comes from a reliable source and is scanning normally.
- Daily work uses a standard account rather than an administrator account.
- Unexpected links, attachments, popups, and downloads are treated as untrusted.
- Important files have a separate, tested backup.
- Important accounts use unique passwords and multifactor authentication.
- You know how to contact legitimate support without relying on an unsolicited message.
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