This article was published in 2011 and updated in October 2026.
Ask computer users what they fear most, and “viruses” is still one of the top answers. But “virus” has become a catch-all term for any kind of malicious software, or malware. Knowing the difference between viruses, worms, trojans and their modern relatives helps you understand how attacks work — and how to stop them before they damage your system.
Viruses
A computer virus is malicious code that attaches itself to a legitimate file or program. When you open or run the infected file, the virus activates and spreads to other files on your computer. Its effects range from annoying to destructive: corrupted files, crashes or a system that no longer starts. Like a biological virus, it needs a host — and usually a human action — to spread.
Worms
A worm is a self-contained program that spreads on its own, without attaching to other files and without any user interaction. Once on a system, it copies itself and looks for other vulnerable computers on the network or the internet. Because worms spread so quickly, they can cause outbreaks around the world within hours; the WannaCry ransomware worm infected hundreds of thousands of computers in 2017.
Trojans
A trojan (named after the Trojan horse) pretends to be useful software — a free game, a cracked program, a fake update or an email attachment. Once you install it, it does something harmful in the background: stealing passwords, logging keystrokes or opening a “backdoor” that lets attackers control your computer remotely. Trojans don’t copy themselves; they rely on tricking you into installing them.
Modern threats you should know
- Ransomware: encrypts your files and demands payment for the key. It is one of the most damaging threats for both businesses and home users.
- Spyware and infostealers: secretly collect passwords, browser data, banking details and crypto wallets.
- Adware: floods you with ads or changes your browser settings, often bundled with free software.
- Phishing: fake emails, text messages and websites that trick you into revealing passwords or installing malware — today the most common way attacks begin.
- Mobile malware: malicious apps, especially from outside the official app stores, can steal data or subscribe you to expensive services.
How to protect yourself
- Keep everything updated: install updates for your operating system, browser and apps promptly — many attacks exploit known security holes.
- Use security software: the built-in Microsoft Defender on Windows provides solid protection for most home users; keep it switched on.
- Be careful with attachments and links: don’t open unexpected attachments, and check the sender and the web address before entering passwords.
- Download only from trusted sources: official app stores and the manufacturer’s website — never cracked software.
- Use strong, unique passwords and two-factor authentication, ideally with a password manager.
- Back up regularly: keep an offline or versioned cloud backup, so ransomware can’t take your files hostage.
What to do if you’re infected
Disconnect the computer from the internet, run a full scan with your security software (Microsoft Defender’s offline scan is useful for stubborn infections) and change your important passwords from a different, clean device. If the problem persists, or if business data is affected, get help from a trusted computer repair professional. In the worst case, resetting or reinstalling the system and restoring your files from a clean backup is the safest solution.
A. Tatum Jr
Aug 11. 2011
Great break down I like it. Going to add this of my list of things to Tweet.
admin
Aug 11. 2011
Thanks to Karen for this ….
Atish
Aug 11. 2011
Great article. most of the guys only talk about the viruses and all but you have explained here. I liked it.
admin
Aug 11. 2011
Yep that’s true … Anyways thanks for the comment
Suraj
Aug 11. 2011
Very nice.
Thanks for sharing this article!
Tony
Aug 11. 2011
I hate these things.
sibin
Aug 12. 2011
Thanks .Nice info about Viruses.Please publish a post about How to remove virus manually….I think you can….
admin
Aug 12. 2011
Lets see then … Thanks for stopping by Sibin 🙂
Raj
Aug 12. 2011
this post will clear most of the confusions people have about Trojans, worms & viruses… nice insights..
admin
Aug 12. 2011
It will … Thanks for the comment Raj 🙂
Ganesan K
Aug 12. 2011
Thanks for sharing a quick definition of virus and trojans.. Keep posted buddy..
Thanks,
Ganesan K
admin
Aug 12. 2011
Looks like you’re visiting this blog for the first time … Thanks for stopping by and commenting 🙂
Eric Murphy
Aug 12. 2011
thats a nice explanation about viruses and worms. I love it.
Priyangshu
Aug 13. 2011
A nicely explained differenciation dude !
Venkat
Aug 22. 2011
Nice post with brief yet clear explanation. It would have been better if even rootkits, malware and spyware were added.
Venkat
admin
Aug 22. 2011
Thanks for stopping by and commenting …. Will see if I can provide with your requested terms
Rebi from
Aug 27. 2011
Clearly explained about viruses,helped in many confusion….
admin
Aug 27. 2011
Good to know that you found it helpful … BTW thanks for stopping by and commenting 🙂
Sandipan Mukherjee
Sep 09. 2011
I already knew about them. However it will be useful for people who are not from computer background or who do not have idea about it. Great job dude.
Jeane
Jan 30. 2012
That was really gold mine article. This headaches have been part of the online community and having a counterfeit for this one would mean a great margin for blog efficiency and ones machine of course. 😀