It is one of the least publicized achievements of the computer revolution: a huge, arching communications network connecting 60,000 computers by high-speed data links and ordinary telephone lines. Developed by the U.S. Department of Defense's Advanced Research Projects Agency in the late 1960s, Arpanet, as the information grid is called, has carried everything from unclassified military data to electronic love notes sent from one lonely researcher to another. But last week it became the conduit for something much more dramatic: one of the most sophisticated and infectious computer viruses the world has yet seen.
The trouble surfaced in computer centers at...