Why oh why do so many people insist on perpetuating myths when it comes to Linux? The amount of Fear, Uncertainty and Doubt that is spread around when Linux is mentioned is amazing, and much of it is spread by people who should know better, or by those who will blindly parrot what they have heard from others without checking the details.
It really annoys me when people say that the only reason Linux does not suffer from viruses is because it is not widely used. I'm sorry, but if your definition of not widely used includes the majority of Web servers globally, a growing share of application servers and a small but significant desktop market, not to mention the embedded Linux kernel found in everything from routers to phones, then you might at least be half right, but the security of the Linux OS depends on many factors, and obscurity is only an extremely small subset of one of these factors. The fact that people do not hate Linux as much as they hate Microsoft is much more significant than the number of users Linux has.
By design, Linux is more secure as an OS than Windows, and the fact that it's code is open makes it easier for people to find holes and bugs and get them fixed. I'm not saying that Linux developers are perfect, but they have demonstrated a much greater willingness to patch security vulnerabilities that have been identified than Microsoft.
It is great that the Linux community is good at countering the FUD that is spread, through forums, blogs and IRC channels, and hopefully this can continue as Linux becomes more and more mainstream.