In the most part, Linux is engineered in a fashion that makes it hard for viruses to run. Also, because more PCs currently run Windows, it is more worthwhile writing viruses for the Windows platform. However, there are many reasons you might want a virus scanner on your Linux PC:

Estimates vary, but generally it is believed that there are 100 to 500 Linux viruses out there. The tiny number of Linux viruses that do exist have never resulted in a significant outbreak. In comparison to the plethora of viruses and worms in Windows-based platforms, the volume of Linux viruses is insignificant. So this leads us to two questions: why are there so few Linux viruses and are Linux anti-virus tools necessary?

There has been a lot of debate recently over whether or not it's a good idea to run an anti-virus program if you're using Linux. I hope to clear up a common misunderstanding: An anti-virus program running on Linux is designed to detect Windows viruses, so that they aren't spread, unknowingly, by the Linux user. This article explains when you should run a Linux anti-virus, and what exactly they do.

If Linux is hardly affected by viruses, why do system administrators use anti-virus software on their Linux email servers? Because an anti-virus scanner on a mail server can serve as another level of defense for Microsoft Windows desktop users.

Viruses. They happen to almost everyone. If they don’t happen to you, it’ll be your cousin Stan who get joins the malware-of-the-month club, and you’re the only person he knows who can fix a computer. The problem is, things are so screwed up that Windows won’t even start properly.

Often, even if we do catch a virus, it's not so difficult to eradicate it using installed anti-virus—but if your system has been crippled, try using Linux to scan the drive for viruses instead. As any Linux veteran knows, one of Linux's greatest uses is fixing unbootable drives—recovering files, deleting files, and even killing viruses.

Historically, there have been some [gnu/linux effecting viruses]. And there are browser exploits, of course, that no operating system is completely immune from. However, viruses, as we think of them in the Windows world, are highly unusual.

Anti-virus companies play a losing game. Casting their nets wide, they catch common, malicious viruses and worms (known to the industry as “malware”), but it may take days before their software updates can prepare your computer for the next attack. By then it could be too late.

When a seasoned Windows user first migrates to Linux, the first question is always “where is the anti-virus?” I have been asked this question countless time and were always given the “you are lying to me” kind of look when I told them that they don’t need anti-virus software in Linux.