Mitglieder.gen is not a specific Trojan, but a generic detection for future variants of the Mitglieder family. This group of Trojans downloads files from web pages and then, runs them.
Such files can be of any nature, including malware.
Mitglieder.gen does not spread automatically using its own means. It needs an attacking user's intervention in order to reach the affected computer. The means of transmission used include, among others, floppy disks, CD-ROMs, email messages with attached files, Internet downloads, FTP, IRC channels, peer-to-peer (P2P) file sharing networks, etc.
However, other variants can be distributed via email in two different ways: they can be sent massively or by certain variants belonging to the family of worms Bagle.