If you don't download and install programs randomly from the Internet, but use the Software Manager, you are not very likely to get a virus: http://www.howtogeek.com/135392/htg-exp ... en-you-do/
. However, on the Internet you are still susceptible to web threats: http://en.wikipedia.org/wiki/Web_threat
. Even if you use a sandbox! Because you
as a human are the least secure thing to use to browse the Internet with
You can be tricked and fooled by social engineering, phishing, spam, and other ways to get your personal information for some malicious use. A virus or other malware isn't always needed for that. If you are very concerned about that, you can look into add-ons for your browser like NoScript, which blocks a lot of web threats.
If you want a virus scanner, install clamtk. That is a free and open source virus scanner, though like all virus scanners for Linux you would use it mostly to catch Windows viruses so you don't pass these on to others through email attachments and such.