It's all in the design.

D Wayte: "Maybe nobody writes viruses for the Mac/Linux because they require so much effort???"

And quite probably because very few (read 'almost none') of the few Linux viruses that have been written can spread beyond what the infected user can write to (e.g. Their own files, network shares, etc...)

Such is the design of UNIX(tm) (which is what Linux, etc... is based on.).