Why was there a large number of virus’s on Mac’s running OS9 when their market share was way less than it is now?

Lev is correct.

The truth is, it's a little of both. Mac and Linux certainly do have vulnerabilities, but not nearly as many as Windows. The idea that Macs are simply not common enough to be worth exploiting is pretty thin at this point, since Mac OS X has gained quite a bit of market share. However, certainly more energy has been put into developing Windows viruses, so comparing the number of viruses on each platform isn't a great way to compare the inherent security of the OS.

It's also worth mentioning that many security holes are found in third-party software which is shared across platforms, like Firefox, Adobe Reader and Java.

So far there have been zero instances of malware for OS X with the ability to propagate effectively, and every instance has required the user to explicitly enter their administrator password.

It's certainly true that ANY platform can host a trojan horse, and this is NEVER going to change as long as it is possible to write programs for a platform. That much should be common sense.

The truth is, it's a little of both. Mac and Linux certainly do have vulnerabilities, but not nearly as many as Windows. The idea that Macs are simply not common enough to be worth exploiting is pretty thin at this point, since Mac OS X has gained quite a bit of market share. However, certainly more energy has been put into developing Windows viruses, so comparing the number of viruses on each platform isn't a great way to compare the inherent security of the OS.

It's also worth mentioning that many security holes are found in third-party software which is shared across platforms, like Firefox, Adobe Reader and Java.

So far there have been zero instances of malware for OS X with the ability to propagate effectively, and every instance has required the user to explicitly enter their administrator password.

It's certainly true that ANY platform can host a trojan horse, and this is NEVER going to change as long as it is possible to write programs for a platform. That much should be common sense.