Generally, the way these viruses propogate themselves is by making a change to your email program. If you are infected, you may not even know it. But, everytime you send an email, you are unwittingly sending the virus along with it. So it does not matter if you know the sender or not, you could be getting a virus every time you open an email attachment. I would definitely not open an attachment to an email if I did not know what it was for, regardless of who sent it.