填空题

Viruses are human-made software programs 【B1】 specifically to wreak havoc on personal computers and networks. The 【B2】 of contracting one of these computer viruses over the Internet has increased dramatically. Typically, you get a virus by opening e-mail 【B3】 . Some viruses are relatively 【B4】 to individuals. They just 【B5】 themselves to 【B6】 messages or e-mail themselves to all the people listed in your address book. The sudden flood of email 【B7】 mail servers, causing the system to 【B8】 . Other viruses are more 【B9】 ,and may lie asleep until a certain date. Then they 【B10】 to life to do their dirty 【B11】 . Sometimes a strange message appears on your screen, or data and programs may be 【B12】 . In the worst case, all the contents of your hard drive may be 【B13】 out. These 【B14】 programs start on one computer, then replicate quickly, 【B15】 other computers around the world. In 1988, a student at Cornell University sent out a virus by 【B16】 , nearly 【B17】 the Internet to its knees. More recently, the "I Love You" virus caused over $1 billion in lost 【B18】 as it 【B19】 e-mail systems worldwide. Last year 【B20】 , 10,000 new viruses, worms and Trojan horses were unleashed.