Most viruses (or viri) are not so much written as rewritten - there are lots of them wandering around and any idiot can take them and change a couple of things and set them off. But most of them are written in Visual Basic, as far as I know, and target PCs and Outlook Express.

There are two kinds of virus the Mac gets: the AutoStart worm, to which the solution is not to set your CD player to autoplay CDs, and Word macro viruses, for which fixes can be downloaded.

Yes, Macs do get software later. They also get worse software in many cases, for example most Mac printer drivers lack the handy features that are the norm in PCs.