Rob Griffiths has an excellent piece on Macworld regarding the Leap-A malware which could infect your Tiger-based Mac, if, well, if you're either not paying attention or are just stupid. Mark Allan has what should be the obvious, common-sense approach to not getting infected:

1. Are somehow sent (via email, iChat, etc.) or download the "latestpics.tgz" file
2. Double-click on the file to decompress it
3. Double-click on the resulting file to "open" it

...and even then, most users must also enter their Admin password.

You cannot simply "catch" the virus. Even if someone does send you the "latestpics.tgz" file, you cannot be infected unless you decompress the file, and then open it.