What happens if you discover a folder on your server that has a bunch of spammy content that has no relevance to your site whatsoever? Typically, Google will deindex your site when it discovers this content. Therefore, you need to be on top of the game and ensure that it doesn't happen.
Unfortunately for someone on Google Groups, it did.
Bergy of the Google Webmaster Central Team, therefore, offered advice on what you should do when you are afraid that your rankings will drop, or worse, if you'll be deindexed. He suggests the following:
Delete the spammy content: If it's not relevant, you don't need it.
Get Google to reconsider your site for reinclusion: Simple and self-explanatory. You don't want to be deindexed because of being an innocent victim. Google has more information about submitting a reinclusion request.
Check with your web host: Make sure that everything is current on your host, including control panels, bulletin board software, blog software (Wordpress users have been hit really hard lately!), and any other applications that may require updating.
Forum discussion continues at Google Groups.