n the first half of the 1980s, computer viruses -- programs that reproduce themselves by "infecting" other programs -- existed mostly in labs. A few had managed to find their way into the wild on the Apple II platform, but for the most part they were tightly controlled by computer researchers.

And then came Brain. Discovered in the first weeks of 1986, it was the first PC virus to widely circulate in the wild. Distributed via 5.25-inch floppy diskettes, Brain's spread was extremely slow by today's standards -- but it unleashed a tide of ever-more harmful viruses, worms, and other malware that we're still struggling with 20 years later.