MALWARE, THE MENAGERIE of malicious software that includes Trojan horses and worms, first made its appearance in the early 1970s, before personal computers had entered the public consciousness. A self-replicating program called Creeper infected the ARPANET, the forerunner of the Internet. This virus was not malicious—it simply printed on a screen, “I'm the creeper, catch me if you can!”—but it triggered the first antivirus program, Reaper, which removed it.

Viruses went public in a big way with the proliferation of the personal computer during the 1980s. The first PC virus, Elk Cloner, infected early Apple computers. In 1986 a virus called Brain emerged on PCs that booted up with Microsoft's disk operating system, spreading via floppy disks.