Threat chains are chain letters that threaten to do something to you, your computer, or to someone else if you do not pass on the chain. The groups making the threats generally claim to be able to track who you send e-mail to so they will know if you have passed it on or not. E-mail tracking, which is generally not possible, is also claimed in many of the Give-Away Hoaxes
Threat Chains
Not a Joke Virus Threat
AOL Riot June 1, 1998
MSN ? . Hacker List Threat Bad Babysitter Chain
Ayumi Tachibana Chain

From:http://hoaxbusters.ciac.org/HBThreats.shtml