Lehigh was a virus program that attached itself to an important DOS program called COMMAND.COM. There were two parts to Lehigh: the worm, which spread the virus to other PCs through floppy disks, and the bomb, which damaged data on the hard disk of the infected system. The worm operated quietly in the computer's memory, scanning any floppy disks inserted into the disk drives. If a copy of COMMAND.COM was on a floppy inserted in the drive, Lehigh's worm quickly inserted a copy of itself into COMMAND.COM, "infecting" that floppy disk. Lehigh kept track of the number of copies of COMMAND.COM it had infected, waiting until it had infected COMMAND.COM on four floppies. Once Lehigh infected the fourth floppy, the bomb portion of the virus is activated. The bomb program was a simple one: it erased the directory of the infected computer's hard disk, effectively destroying all data on that hard disk. Most disk containing Lehigh were ferreted out and erased, but the incident remains something of a watershed in PC history; it was the first really nasty widespread virus.