Network servers present an interesting problem. They can support a wide variety of machines, but may run an entirely different operating system. For instance, a UNIX server may support a network of PC and Macintosh workstations.
The UNIX system cannot be infected by the Jerusalem-B or WDEF viruses, but infected files may be stored on its disk. Once the network server has infected files on it, the workstations it supports will rapidly become infected as well.
Since the viruses never execute on the server, the administrator is limited to static detection techniques such as scanners or change detectors. The nature of network servers allows these tools to be run automatically during off-peak periods.