Computer Virus
In plain terms
In everyday use, "virus" refers to any piece of malware. Strictly speaking, it's a precise category: a program that copies itself into other files to spread.
Definition
A computer virus is malicious software that reproduces by inserting a copy of itself into other files or programs, and that spreads when those files are opened or shared. In everyday language the word covers malware as a whole; this entry gives both meanings and points to the exact category in each case.
How it works
Strictly speaking, a virus needs a host: it attaches itself to an existing file, and its execution depends on that host being opened. This is what sets it apart from a worm, which moves on its own, and from a trojan horse, which doesn't copy itself. Historically it spread through floppy disks and shared executable files; today the most common form travels through office documents containing macros, which is why office software asks for confirmation before enabling active content. That said, if you're searching for "virus" after noticing a problem, the question that actually concerns you is rarely about terminology: what you observe points to a specific category. Files that no longer open with a payment demand: that's ransomware. A hot, slow machine for no reason: that's cryptojacking. Passwords leaking without you clicking anything: that's spyware or a keylogger. An alarming window selling a cleanup tool: that's scareware — and often, there's no infection at all.
Warning signs
- Office document asking you to enable macros or active content to display
- Files or shortcuts appearing in folders you didn't touch
- Programs launching on their own, or repeated error messages at startup
- General slowdown with no identifiable cause and no change in use
- Contacts receiving files from you that you never sent
How to verify
Start from what you observe rather than the word: the symptom points to the category, and each category calls for a different course of action. An up-to-date antivirus scan is a reasonable starting point, provided you know what it's worth: a detection is information, an absence of detection is not a certificate of good health.
What to do
Don't enable macros in a document you received, unless you were specifically expecting one from its sender and confirmed it through a channel other than the message itself. Keep the system and software updated, and keep regular backups disconnected from the machine.
If it already happened
Disconnect the device from the network and first identify what you're dealing with, starting from the symptom: the response to ransomware has nothing to do with the one suited to spyware. Stop sharing files from this machine until the situation is established. Report the incident to cybermalveillance.gouv.fr, which will direct you depending on the case.
Frequently asked questions
- Can my phone get a virus?
- Strictly speaking, spreading by infecting files doesn't match how modern phones work, where each app is isolated from the others. What does exist, though, are malicious apps and excessive permissions — that's where to look, rather than under the word "virus".
- Is free antivirus software enough?
- The more useful question isn't the price but what you expect from it. Antivirus software stops what's already known; it doesn't replace updates, disconnected backups, or only installing from original sources. Those three measures prevent more situations than a choice of product.
- How can I tell which type of malware it is?
- By what you observe. Locked files with a ransom demand: ransomware. A hot, slow device: cryptojacking. An aggressive alert selling you a solution: scareware. Compromised accounts with no click on your part: spyware or a keylogger. Each of these entries gives the matching course of action.
Related attacks
Official sources
Also known as: virus, computer infection, PC virus, catching a virus.
This article is part of the Malware family. Last updated: 2026-09-03.