Software which infects other programs and may destroy information. A virus insinuates itself into executable files or documents, from which it uses the computer’s links to other programs to replicate itself. In contrast, worms are able to spread to other computers and replicate themselves there, using e-mail address lists, so that a receiver is apt to believe that he knows the sender and thus is apt to open and activate the worm. A Trojan horse is a type of virus that, when downloaded or opened, accesses the computer’s vital systems and damages them. A hoax is an e-mail posting that raises an alarm about a nonexistent virus. Unaware readers are apt to forward the information to others, creating a chain-letter effect.