I am trying to fix my husbands computer - mac osx 10.7.5  We got attacked by a VIRUS that affected my pc, my ipad, my samsung galaxy s3 phone and his mac.  It kept asking us to update flash player - and would not let us get onto gmail at all.  it is on a loop.  I have fixed my pc (after much effort) and have reset the router.  I have fixed my ipad.  But his mac is defeating me.  Any suggestions?  Sophos does not FIND it when I scan with that, and I tried a scan with ClamXav which did find a trojan and quaratined that...but it has made no difference.  I have cleared cache and browser history.   I can't remember everything ELSE I have tried.  Suggestions please.

Juliet You may have to buy a virus scanner for the Mac.http://www.adobe.com/software/flash/about/

On these pages, Adobe records the latest Flash Player version info for many o.s. platforms. The page also shows your present version info. It will show you if you actually need to upgrade your current application.

Where to Get Trusted Adobe Upgrades?

Because malware writers will often use strategies that are crafty to get malware LOOK like something you need to trust, it is crucial that you get your upgrades just right from the seller that releases them. You can always visit Adobe's website directly for a Flash Player upgrade.CERIUM, good post. 

And it also applies to other OS.
There are malware that imitate the Flash logo and look like the real thing.
As Cerium said, always go to the Adobe site first.