The MyDoom computer worm, which first appeared Monday afternoon, spread rapidly by e-mail and infected vulnerable systems at an amazing rate.

Mac users were bombarded with infected messages, just like everyone else, but no harm was done. Even Mac users who fell for the trick and clicked on the attached file were not infected. The worm won't run on Macs.

Windows worms and viruses affect Mac users in the same way that bullets affect Superman. They're annoying, but they don't hurt.

Apple does not claim that Mac systems are immune from viruses. In fact, the company actively promotes McAfee's Virex software as part of the .Mac service. It's also true that there are "macro viruses" that can infect Microsoft Word on the Mac.

However, the fact remains that there are no viruses that target Mac OS X. Zero. Zilch. None.

Is this because Mac OS X is more secure than Windows? Could be.

There's a more popular theory, however. The "Bigger Bang for Your Buck" theory (aka "Too Small To Bother") goes something like this:

"Virus writers don't bother with Macs because there aren't enough of them. If 95% of computers ran Mac OS X, the virus situation would be just as bad."

That's just wrong. If 95% of computers ran OS X, virus writers would face a much bigger challenge than they do today.

How is Mac OS X more secure? Let me count the ways:

SECURE BY DEFAULT
Mac OS X ships with most Internet services and sharing features turned off by default. It is more secure from the very beginning. A brand new PC running Windows XP is at least three steps away from basic security.

UNIX FILE SYSTEM
Like Linux and other UNIX-based systems, Mac OS X gives limited access to each person using the system. A careless user could wipe out his own files (accidentally or on purpose), but he can't damage other users' files or monkey with system files. Windows users can make a mess of everything on their system with very little effort (and often do).

PASSWORD PROTECTION
Mac OS X requires an Administrator password to change some settings, to update system software, and to install new programs. Windows has Admin accounts and passwords, but permits software to run and install new programs without password protection.

GOT ROOT?
Some files in UNIX-based systems can only be accessed by the "root" user. This all-powerful user account can modify files and settings that other users cannot.

Windows provides no such protection for system files. Not only do Administrator accounts in Windows XP have the equivalent of "root" access. Programs installed by a non-Admin users can add DLLs and other system files without an Admin password. Furthermore, Windows applications routinely install files in the WINDOWS and WINDOWS/SYSTEM folders, and may even replace system files with newer versions not written by Microsoft.

For now, Windows monoculture rules and Mac OS X is virus free.

It's possible that devastating viruses and computer worms will target Mac OS X in the future, as its market share increases. However, it seems more likely to me that the increased security and better design of Mac OS X will continue to deter virus writers. Even if Mac OS X becomes a bigger target for virus writers, it will never be a better target than Windows.

No wonder Mac users laugh when worms and viruses strike. The bullets tickle.