The MyDoom virus has been spreading like wildfire in the last 24 hours. Some companies are receiving 1000 infected emails a minute. The virus installs a trojan that can leave the host machine open to sending spam or denial of service attacks before it grabs email addresses off an infected computer, and emails itself on. Reportedly, it records keystrokes on affected machines so can mail off passwords, user names etc to whoever coded it.
The worm uses random subject lines that include "test" and "Mail Delivery System" and carries different text messages, including: "The message cannot be represented in 7-bit ASCII encoding and has been sent as a binary attachment." The attachment names also vary, having ".exe", ".pif", ".cmd" or ".scr" as extensions and are sometimes compressed as a Zip file.