Brendy said WYSIWYG sent him a file called Backstreet Boys logg.doc.vbs that was infected with the Sircam worm.

It seems like a file called *.doc.vbs would be infected with a number of old worms. Since Susan has an up-to-date antivirus program, she would have to have disabled it to get the virus. It would have mass mailed itself, but it didn't.
It would have sent its own message, not attach to one Susan sent, but apparently, this one attached to the same clean Virus Alert message we all got.

As for Sircam:
Susan would have had to get infected with Sircam without noticing.
Sircam attaches to files ending in .doc, but it would change it to an .exe file, not a .vbs file
It normally would have changed the e-mail title to be the same as the attachment, but it didn't.
It would have sent it's own message, not attach to one Susan sent, but that's what Brendy said it did.
It would have e-mailed itself to more than one person in her address book, and it didn't.

I personally think the liklihood of any of this actually having happened to be pretty darned slim.