The computer virus is an outcome of the computer overgrowth in the 1980s. The cause of this term is the likeness between the biological virus and the evil program infected with computers. The origin of this term came an American science fiction 'The Adolescence of P-1' written 【S1】______ by Thomas J. Ryan published in 1977. Human viruses invade a living cell and turn them into a factory for manufacturing viruses. 【S2】______ Therefore, computer viruses are small programs. They replicate by attaching 【S3】______ a copy of themselves to another program. Once attached by host program, the viruses then look for other programs 【S4】______ to' infect'. In this way, the virus can spread quickly throughout a hard disk or an entire organization. At some points, the situation will be 【S5】______ totally determined by how the virus was programmed. The timing of the attack can be linked to a number of situations, included a certain time 【S6】______ or date, the presence of a particular file, the security privilege level of the user and the number of times a file is used. So-called' benign' viruses might simply display a message, like the one that infected IBM's main computer system last Christmas with a season's greeting. Malignant viruses are designed to damage the system. The attack is to wiping 【S7】______ out data, to delete files or to format the hard disk. There are two main types of viruses: shell and intrusive system. Shell viruses wrap themselves around a host program and don't modify the original program. Shell programs are easy to write, that is why about 【S8】______ half of viruses are of this type. Intrusive viruses invade an existing program and actual insert a portion of themselves into the host program. 【S9】______ Intrusive viruses are hard to write and very difficult to remove with 【S10】______ damaging the host file. 【S1】