Most system administrators are incredibly promiscuous with the root account: they will download software from the Internet, compile it, and install it --- all as as root. And these days, with more and more Linux programs being distributed as binary RPMs, many system administrators don't even have the source-code for the programs that they are installing.
...what's stopped the spread of viruses on the Linux platform isn't technology, but the lack of interest from the virus writers. Why write a Linux virus when the same skills will let you bring up a new web-site and become a millionaire in just a few weeks? But if the economy goes south, we're likely to see a suddenly bloom of viruses from out-of-work overachievers.
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