(The following question is from a Windows user that doesn't know a lot about computers. Please respond to his question as gently as possible.)
As the price of Macs come down and they become more common, won't they end up with a lot of the same problems Windows? Viruses and that sort of thing? It's my understanding that the people that create viruses stick with PCs because there are so many of them.
The Mac operating system is built on the UNIX Kernel, which is one of the oldest and most secure operating systems available. Most tools and scripts for creating viruses are for Windows, and most virus authors are more familiar with Windows and the UBM platform than they are UNIX. And, there are more Windows users, so they are more appealing target, since most viruses depend on large numbers of infections to really accomplish anything.
Because of that, most Mac users don't run antivirus software. Most Mac users are, however, in denial. There's already a nasty Mac trojan in the wild that's hidden inside pirated versions of iWork 09 that people download via Torrents and get infected with. I won't go into why it's stupid to download iWork via P2P networking since you can download a free 30 day trial from Apple, but it illustrates how things are begining to happen on the Mac.
Malicious code hidden in Web pages are begining to target Macs, as well. A couple of years ago, after Apple announced that OS X was bullet-proof (even if it was, why invite bazillions to prove you wrong?) a computer researcher released three harmless worms into the wild to prove to Apple the weaknesses that OS X had. Apple immediately asked him not to release the source code until they had the patches in place.
But OS X is based on the FreeBSD UNIX platform that was written from the ground up to make networking secure, and it's been battle tested by a lot of computer geeks. It's proven to be far more secure, out of the box, than any Windows machine. It's not invulnerable, but it's not all that inviting of a target. Still, now that it's runnning on the Intel chips instead of the PowerPC chips from IBM and Motorola, more and more hackers will be able to exploit any weaknesses it has.
But for the most part, Mac users really don't have to worry very much about viruses. At the same time, they shouldn't become overconfident, either.