I can't think of any way that this would cause a virus to be sent, as the announcement list feature doesn't even support attachments. My guess is that the virus checker in question is a bit hyperactive, and/or flags messages based on criteria other than attachments to the message itself.
You may want to check with the administrator of the recipient's mail server to see if there is any sort of logging with their virus checker. It may provide more details.
- Jeff @ DreamHost
- DH Discussion Forum Admin