At ubuntu.com there is this "only in ubuntu" that says "Built in virus protection":
What is the Ubuntu built it protection? What is the program in charge of this and how does it work?
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"Built-in virus protection" is a simplification of the security features of Ubuntu.
Ubuntu requires applications to be run as super-user to cause any damage. It also includes AppArmor to supplement that.
There is also the safe and secure repository model which gives you access to thousands of applications through the Software Center which are tested by package maintainers.
Since it is free software more people have access to the source code and according to Linus's law: "Given enough eyeballs, all bugs are shallow", which means that
Given a large enough beta-tester and co-developer base, almost every problem will be characterized quickly and the fix will be obvious to someone.
The security exploits are quickly patched up and delivered to you through the Update Manager.
My 2 cents are that it is possible to get a virus for Ubuntu, but:
I have had discussions with some people who claim that the Linux population makes it a less favorable target for viruses.
There are a number of things about Linux and other Unix based platforms that make them not pleasant environments for viruses.
In whole and in part these factors, make introducing viruses more difficult, easier to detect, and easier to disable.
Excerpt from Psychocats Tutorial Website:
Conventional wisdom in the Linux community says that there are either no or very few Linux viruses out in "the wild," and that most are just proof-of-concept theoretical viruses. Some people recommend installing a virus scanner like ClamAV in order to protect your Windows-using friends from Windows viruses you might accidentally send them. I don't really see how that's an issue, though. If you have an attachment you created in Linux, why would it have a Windows virus in it? If your computer has been compromised in such a way that you don't have control over what you send other people, then you have a lot more to worry about than spreading viruses to your Windows-using friends!