"A creature composed of light would be one fragger of a virus." he said, thinking out loud. "It wouldn’t be affected by any intrusion countermeasures, since they’re set up to attack the deck or the decker. It would be nearly impossible to detect, because it wouldn’t interfere with the other data transmissions. Light doesn’t interfere with itself unless the two beams are exactly in phase—that’s why a fiber-optic cable can carry thousands of commands and transmissions simultaneously. One more pulse of light down the tube wouldn’t affect it a bit. And it wouldn’t hurt the hardware—at least, I don’t think so. But there is one thing the spirit would do—it would sure mess up stored data."