The reported Flashback infection / virus for Macs is another example of knowing what you should install on your computer and being familiar with legitimate downloads for your computer.
Looking at the image above you should know NOT to install this as this is not what appears on your Apple computer for updates.
Another point to remember is that no antivirus software for macs would have stopped this as they are not updated.
There are security updates available from Apple through software update to patch this vulnerability.
The easiest way to find out if you have been infected is to run the tool from Github.
Then hopefully you will get an image like this after running it.
Apple have now released their own patch which will remove the Flashback Trojan
Here are some additional links to help you.