i agree for the most part with JoeUser11 - Linux itself is quite robust. However, you may have friends, family and colleagues who use Windows, and if you're exchanging files with others, it can be handy to screen them for viruses, for the benefilt of Windoze users. To that end, there are various antivirus software apps available for Linux - eg, Clamav.
The other point I'll make here, with rueful bitter experience, is that whilst Linux itself is indeed fairly secure and robust (and certainly much better than Windows), the user still needs to take responsibility for basic security - eg, firewalling. Using any computer, whatever the OS, on a fast internet connection with no firewall is asking for trouble - I know, because it happened to me, running Linux, and I was hacked... not the fault of the OS - I was just being stupid...