How Do Viruses Work


If you’ve ever experienced a computer virus, you may be wondering what exactly it is and how it works. We’re going to give you a general definition, and an overview of how a virus can totally ruin your day.

A computer virus is a small software program designed to spread from computer to computer, interfering with your system’s operation. It can do this in a number of ways, including: corrupting or deleting data, using your e-mail to spread to other computers, erasing everything on your hard drive and more.

Here’s the way most viruses work

An infected program is run – this can be either a program file or a boot sector program. For example, if a virus is embedded within a Word document, it can be activated as soon as that document is opened. But if the “NORMAL.DOT” template (the most common target for viruses like these) is infected, the virus could be activated as soon as Word is started up – regardless of whether or not you’re opening a specific document.

The infected program is modified – the virus code runs instead of the proper program code. Usually, the virus will modify the first few instructions to cause the code to “jump” to where it’s stored, where it will then begin to execute.

The virus code becomes active and takes control of your PC – there are two ways a virus will behave when it’s run. “Direct-action” viruses will immediately execute, seeking other programs to execute and/or exhibiting whatever malicious behavior their author coded into them; most file-infector viruses are this type. In contrast, “memory-resident” viruses don’t do anything immediately. Instead, they load themselves into your memory and wait for a triggering event that will cause them to “act.” Many file infectors – and all boot infectors – do this; boot infectors generally have to become memory-resident, because when they’re executed the system is just starting up and there isn’t much they can do immediately.

What the virus does depends on what it’s written to do. However, their primary goals include replication and spreading, so they’ll usually search for new targets to infect. A boot sector virus, for example, will try to install itself on hard drives, memory sticks (like USB sticks), CDs, or similar things it finds in the system. In contrast, file infectors will usually stay on your computer, in its memory, and look for programs that can be targeted for infection.

Malevolent viruses which damage files or wreak havoc in other ways will usually act on triggers. These can be things like activating only on particular days of the year (like the “Friday the 13th” virus), or have a random trigger, like deleting a file when it’s been run 10 times. Some viruses, however, do nothing other than try to get to as many files and systems as possible.

Virus Prevention

Today, there are many “worm” viruses that travel the internet, actively seeking vulnerable computers; your computer has the potential to be infected simply by being online. Luckily, even though new viruses are being invented all the time, there are some easy steps you can take to protect your computer. The most important, of course, is installing and regularly updating your virus protection software.

Although anti-virus software is absolutely critical, it’s often not enough to provide maximum protection when it’s used alone. You should also be regularly updating your operating system. On Windows, you would use Windows Update – open your Control Panel, click “System and Security,” and you’ll find Windows Update there. However, if you’re using Windows 10, your system will automatically install and download essential updates for you, making the process easier. Mac users can install updates by clicking the “Apple” menu and choosing “Software Update.”

Other Tips on Virus Prevention

  • Don’t open emails with attachments if you think the message is suspicious – even if they’re from a friend or acquaintance.
  • Don’t download files from strangers.
  • Be extra cautious when downloading files from the Internet.
  • Avoid subscribing to e-mails from groups or informational websites, as these e-mails are considered spam.
  • Use one of the antiviruses listed on to guard against viruses, spyware, and other potentially dangerous software.

Advertiser Disclaimer: We are a professional review site that receives compensation from the companies whose products we review. We test each product thoroughly and give high marks to only the very best. We are independently owned and the opinions expressed here are our own. We are not responsible for direct, indirect, incidental or consequential damages resulting from use of any antivirus software and/or this website.