The late 90s-early 2000s were not only the golden age of the internet, they also seemed to be the golden age for malware. Over that time period, few viruses were able to match the reign of Klez.
Like many other viruses of its time, Klez spread through email. Users were duped into opening infected files and, once the malware was installed, the victim's address book was opened and copies of the attack were sent to contacts.
Klez, however, took this a step further. Not only did the virus send itself to people in your address book, it also pretended to be from other people. Later, the worm wreaked further havoc by pretending to be its own removal tool.
Klez is a cunning little devil, and variants are still doing the rounds today.