If you are ultra-paranoid then you can download the source code, inspect every line of code yourself, and then use a compiler that you have written yourself from scratch ( do you know for sure that visual studio or a binary distribution of gcc doesnt embed viruses in compiled code? No!! ) in a faraday cage to compile it all into a installable game that you can be utterly certain is virus-free.
So in a nutshell you have more assurance that planeshift is virus-free than 99% of all other software you are likely to have used. And thats a good thing

Though there were those rumours of a virulent plague conjured up with the most foul of Dark Way arcane powers and spread by a fiendish legion of cake-bearing evil ones... but those are purely unfounded rumours and wild speculation and should not be heeded by anyone with the sense to know whats good for them.