there are virus for linux, it is not imune from viruses. There is just not many cause it is not the most used platform. If new software was never deleveloped, and current software was free of bugs, and if ever user made sure they updated there system all the time, then it might be possible to make virus redundant.
THIS WILL NEVER HAPPEN............
how many people are updating outlook to protect against virus, how many people can afford to always be upgrading, especially when the machine already does what they want. When are peopel not want to something new. If new things are being developed then there will always be bugs.
There will always be script happy kids, there will always be a need to protect your computer. People will manually execute things them self.