I had someone or something send out emails to everyone in my Yahoo contact list. Is this a virus or what?

I first became aware of this when I had a whole bunch of mailer daemons in my spam box. Then a friend emailed me back and asked me if I sent this to them. It was an ad for some vacation spot. Not from me. I use AVG Free for virus checks and can't find anything. Is this an issue with my Yahoo account or with my computer? How do I get rid of this? How do I keep this from happening in the future?