Why Did My Computer Get a Virus?

Category: Security

For the third time this month, my computer has been infected with a virus. My wife and kids swear they didn't click on anything suspicious. I have anti-virus protection, so how does this happen? Also, any insights for prevention would be appreciated.

How Do Computers Get Infected?

There are many ways a computer can become infected by a virus, Trojan, spyware, or other malware. Many of them depend on your cooperation, or at least your inattention. Below are some of the most common ways that your computer can get infected, and suggestions for preventing it.

Clicking without questioning is one of the best ways to get a virus. Whether browsing the Web or installing new software, many people just "follow the prompts" given to them by a Web site or installation program. Malware distributors take advantage of this mindless behavior, prompting users to ploys that seem crude but actually work quite often.

A favorite trick is to pre-check "permission" buttons in installation programs, implying that the "default" thing to do is accept whatever malware-laden toolbar or add-on program that is offered.
Think Before You Click

Scare tactics are often used to induce hasty clicks. A pop-up window may scream, "YOUR PC IS INFECTED!" and urge you to click for a cure. Often the "cure" is really the disease, which did not exist on your PC until you downloaded it by clicking. "This Web page requires the Doohickey plugin; click here to install" is another con that malware pushers use. ("Doohickey" is a made-up name, not a real plug-in.)

Attachments and Free Stuff

Clicking on email attachments is another way to activate a spyware or virus. Curiosity often plays a role in getting people to click on attachments from unknown senders. Sometimes an email attachment that seems to come from someone you know is actually a forgery.

There is a common misconception that only executable file attachments - those ending in .exe, .com, or .bat - are dangerous. In fact, malicious code can be hidden in files of other formats to exploit vulnerabilities in the programs that open them. Thousands of malware payloads have been delivered via Adobe PDF and Microsoft Office files. If a file can be opened with a click, it can be dangerous.

Pirated music, movies, and software from "warez", "file sharing" or "torrent" sites often contain hidden payloads of malware. If you hang out with dishonest people, it should come as no surprise when they burn you with malware. Similarly, so-called "adult" sites are often traps for the unwary.

Downloading freeware, shareware, and other software from unfamiliar Web sites can bring an infection to your computer. Stick with well-known download libraries, such as Tucows (http://www.tucows.com/downloads) or Download.com, that scan files for malware before posting them for visitors to download.

Sharing files via USB flash drives or CDs is another potential way to pass malware between friends and co-workers.

Not keeping your operating system, browser, and other software up to date with the latest security patches is asking for malware trouble. Even the best anti-virus program can't do its job if you fail to keep its malware signature databases current.

Turning off firewalls or anti-malware software, or never installing them at all, is like wandering into the jungle without insect repellent.

Let's sum it up... Malware comes in many forms and from many directions. Think before you click. Be wary of email attachments. Keep your software up to date. Always keep your firewall and security defenses up. Stay out of the shady parts of the Internet. Following these tips, and giving regular reminders to others who use your computer, will go a long way toward keeping you virus free.

Here's a free poster with my "Five Tips to Avoid Computer Viruses" that you can download and hang on the wall near your computer.

Your thoughts are welcome on this topic. Post your comment or question below...

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Most recent comments on "Why Did My Computer Get a Virus?"

Posted by:

12 Jul 2011

As much as I hate to admit it, I actually sucked for one of those scare tactic on-screen virus "warnings" because it looked exactly like the reaction my malware software posts if I click on an unsavory web site. That's no excuse. I should have stopped and looked closely to be sure...

To make a very long story, short, it took me five days to finally rid my computer of the malware and I'm a fifteen year veteran user.

Heed Bob's words...and mine, too!

Posted by:

Jim Dobbins
13 Jul 2011

Maybe they didn't actually get rid of the bug the 1st time around??

Posted by:

13 Jul 2011

I'm guessing the person asking the question did a full scan with his AV of choice. Since no AV product will be 100% effective 100% of the time, he might consider a few free on-demand scanners to see if they find something his AV missed.

Microsoft System Sweeper - create a bootable disc on a known clean computer. If the infected machine is 32-bit, create the 32-bit disc. If the infected machine is 64-bit, create the 64-bit disc. Booting from the disc bypasses the usual system startup where many viruses and spyware try to hide.

Malwarebytes - a go to scanner that finds a lot of malware that traditional AVs miss.

Microsoft Safety Scanner - another tool using different algorithms to find problems.

Posted by:

13 Jul 2011

Hi Bob,
All downloads MUST be checked before running. I download a trial game from Download.com a few years ago now. When I ran the program, my antivirus said that there was a virus. I uploaded the program to virusjotti where it was scanned with about 20 different AVs. about 10 came back positive. I sent an email to download.com saying that there was this virus and gave them the virusjotti link. After a few days I got a reply saying that they had tested the file and that for "THEIR" scanner it was clean. I have no idea how they could say that it was clean with "THEIR" scanner but another 10 or so scanners said it was infected.

Posted by:

13 Jul 2011

Another source of malware comes from Facebook. Avoid anything that looks odd or makes little sense to you. It has frequently been the source of infections on several public PC's I maintain. I have since switched them to using Microsoft's freebie called Steadystate and it has kept them clean. But it's not a real good choice for a home PC. Another suggestion is to make sure that all users are NOT Administrators on the PC. If they are, create them a "User" level account for everyday use.

Posted by:

14 Jul 2011

I'd like to make one small correction to your article, Bob. Aardvark actually is a plugin for Firefox -- one that I use a lot. And, while it is listed as "incompatible" with Firefox V4 & V5, It actually works just fine if you install the "Add-on Compatibility Reporter" to Firefox as well. Aardvark is an extremely useful tool for isolating and compiling specific areas of web pages for printing. Thanks for your regular informative articles...

EDITOR'S NOTE: Whoda thunkit? I changed it to "Doohickey". For now...

Posted by:

15 Jul 2011

I have been receiving a lot of e-mails that just have a click-able link in them.

Closer inspection of the senders address shows a slight difference ie @aim not @aol

The only thing these friends have in common appears to be Facebook.

Are their Facebook accounts being hacked to get access to their email address book that so many give FB access to?

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