【简答题】

Using international phone links, a group of West German hackers took repeated strolls through NASA computers last summer, as well as through several U.S. military networks. NASA spent three months changing passwords and clearing out "trap door" programs that the intruders had planted to give them access. Another German hacker spent nearly two years cruising through unclassified data in U.S. Defense Department and other research computers around the world until he was stopped last year. And last May, NASA's Jet Propulsion Laboratory in Pasadena, California, was invaded by hackers yet to be identified. Even companies with good security have run into a new problem: the computer virus. Like micro-organisms, these replicate and spread. They're tiny bits of software, often quickly written，that hide in larger programs and then pounce unpredictably.