But experts warn that could change if Linux begins to win a mass audience on the desktop, bringing in millions of users who are less technically-proficient and security-conscious than today's typical Linux user.

"Windows was the only game in town, but now Linux is offering a more tempting prize," said John F. Andrews, president of open-source market research firm, Evans Data Corp. of Santa Cruz, Calif.

Earlier this month, Evans released survey data showing that 11 percent of developers reported seeing malware on their Linux systems, with more than a third of those having three or more infections. While still low compared to infection rates among Windows users, they are the highest totals ever reported in Evans' twice-year survey, which began in 2002.

Earlier this month, a cross-platform virus emerged that could theoretically infect both Windows and Linux. The virus, called Virus.Linux.Bi.a/Virus.Win32.Bi.a, has not been used in any known attacks.

But experts such as Johannes Ulrich, chief technology officer at Bethesda, Md. Internet security group The SANS Institute, say such proof-of-concept code has traditionally presaged the launch of actual malware. "I think we'll see an increase in virus activity as Linux becomes more mainstream," Ulrich said.