This is a virus detection. Viruses are programs that self-replicate recursively, meaning that infected systems spread the virus to other systems, which then propagate the virus further. While many viruses contain a destructive payload, it's quite common for viruses to do nothing more than spread from one system to another.
Indication of Infection
ZIP files sent by Bagle are benign, unless the file within is extracted and executed.
Methods of Infection
Password-protected zip files sent out by Bagle require users to retrieve a password from an email message, and use the password to access a viral file inside a zip archive.