From everything I've read on Apple's discussion forums, the Mac Defender trojan/virus can only be installed on your Mac OS X system under the following circumstances:
- You are using Apple's Safari web browser, and
- the Safari 'Open "safe" files after downloading' setting is enabled, and
- you visit a website with the malicious Mac Defender trojan on it.
Using other browsers, or when using the Safari browser without this setting enabled, it looks like the worst thing that may happen automatically is that the Mac Defender trojan may be downloaded to your system, but I haven't even read any reports about that happening with the Firefox or Chrome browsers. (If the file has been downloaded to your system, but is not running, just drag it to the Trash.)
Sadly, this Safari setting appears to be enabled by default (which doesn't seem like a great decision by Apple).
So, the short story is:
Don't use Safari.
If for some reason you feel compelled to use Safari, make sure the 'Open "safe" files after downloading' setting is NOT enabled.
More Mac Defender trojan/virus information
Here are a couple of links on the Apple discussion forums I've been following:
I hope this Mac Defender trojan/virus information has been helpful. I'll share more here as I learn about it.