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
Word documents altered on the 13th of the month
Methods of Infection
The virus writes its code to the global template, "Normal.dot". Once the global template is infected, all documents used on the system will become infected.