According to FBI files acquired by The Smoking Gun, the bot infected over half a million computers and could sent ten billion pieces of spam per day.

ENGADGET: FBI charges 23-year old Russian in Mega-D spambot investigation

It is spread when the infected USB device is inserted into a Microsoft Windows workstation or server whereby the setup.exe and autorun.ini files run automatically.

FORBES: IBM Distributes Malware-Infected USB Sticks At Security Conference