It used to be that if you were running Linux, you could be reasonablly certain that virus writers and crackers wouldnt come knocking on your network door. Those days are done. Now, crackers are attacking Linux sites,and its only a matter of time until virus writers come calling.
Fortunately, Linux developers and security experts are working to toughten Linux up as Jim Wagner explains in his LinuxPlanet article, which can be found here.