Letum is an Internet worm that spreads via Usenet conferences and by e-mail through bogus messages with infected executable attachments. Once the user executes an infected file, the parasite installs itself to the system and starts a spreading rsoutine. Letum uses own mail engine to send copies of itself to all the e-mail addresses it gathers from locally stored web (.html) files. The worm does not carry any dangerous payload. It can infect both Windows Mobile and Windows systems that have the .NET framework installed. Letum runs on every Windows startup.