GNU/Linux users should not have any false sense of security just based on the fact that viruses designed for exclusively for windows won't run on GNU/Linux. Here is a list of "Linux computer viruses"

Like other Unix-like computer operating systems, Linux is widely considered as secured and well-guarded against computer viruses. Its multi-user environment makes it extremely hard for malware to gain root access to the system files.Though malicious programs that are specifically written for Linux are really small in number, there is still a possibility for them to cause some harm.

If you’ve switched to Ubuntu from Windows, there’s a very good chance that the security failings of Windows featured in your decision. By any measure, Microsoft’s record on security within its products is appalling.

Microsoft security 'vulnerability scorecard' gives false impression of OS security, suggesting Windows is the most secure of all. Of course, start to look behind the pretty graphs and the ugly truth emerges that this is just more Microsoft FUD to try and derail the free software machine.

The idea that there's more security in using less popular software is not only false, but a smokescreen to solving the real security issues

New York, June 15, 2011 – BalaBit IT Security announced an increase of 59 percent on a year-on-year basis in 2010. According to industry and analyst predictions last year there was a slight increase on the IT security market, where the drivers were the compliance related investments.

Organizations of all sizes need to mitigate the risk of insider threats. Misconduct by authorized users represents a grave threat to an organization. According to the 2005 Computer Security Institute and Federal Bureau of Investigation Computer Crime and Security Survey, organizations reported that computer intrusions from inside sources accounted for nearly half of all incidents.

There's no such thing as perfect security. There are no programs that give you absolute software security. After all, security is a process, not a product. Linux's security process, though, is outstanding, which is one reason why it has great security. Here's an example.

If you create user accounts on a server, and then allow people to log on to the ftp server under those accounts, there’s a chance that some hacker could find a way out of the chroot jail. That would allow him to access areas of the file system where he has no authorization, and possibly to plant Trojans or other malicious programs.