Viral advertising. It infects your computer via an email from someone in your contacts. It comes through with no subject line. Whne you open the attached link, the virus attaches itself to your email contacts.
It then perpetuates itself by sending itself to everyone in your contact list, then spreads further when they open it and it infects their contact list.
It gets past your security software when you open the email. It uses different links, so you can't mark it as spam.
The biggest problem is it sends really innapropriate content about Viagra and other such crap to all of your contacts.