Call me a liar, it is no longer virus-proof.
If this is the 1st then it was upto that point. This is just the system files, 3rd party software can also be a security threat (anything that has access to communication hardware).
(in part)
"The first Linux virus first surfaced "in the wild" this year, demonstrating the real fact that protecting a system operating on Linux is now imperative. In order to fairly determine that preventative measures are the best treatment, Linux Computing conducted its own independent anti-virus test for the Linux server version."
Viruslist.com - Anti-Virus Linux Virus-Proof!
The server edition (business) of linux is much more secure. Down to very basic graphics system and very limited commands access.