downloader-BR

Greetings

I decided to do a scan last night and discovered Downloader-BR (which
doesn't seem to have been activated)

I use McAfee v7, with uptodate DAT and engine. I also use Zone Alarm. I also
don't open unknown emails (although OCCASIONALLY other family members use
the computer (with dire warnings of course - BUT....)

Can anyone tell me how this gets onto computers - the McAfee site tells you
stuff about what it is, but not the infection method.

Also, as an incidental - I told McAfee to quarantine the file, and have
since done another scan of the quarantine folder - and nothing is detected.
Does it not detect/bother about files in that folder?

Cheers