People say open source will lead to more viruses and in the short run, I agree, but people dont really think of the bigger picture for some reason.
When Windows gets a virus, you have to wait forever and a day for MS to fix the problem, because they are the only ones with access to the source code that can fix the error in question. With linux, everyone can access the source code, and because of that, anyone can in theory fix the problem. I would imagine linux exploits would be patched up much faster than the same bug in windows