【正文】 atch a virus when you download an infected file from the Inter or copy an infected file from a diskette. Once the viruses is embedded into your puter?s files, it can immediately start to damage or destroy information, or it can wait for a particular date or event to trigger its activity. What are the main types of viruses? Generally, there are two main classes of viruses. The first class consists of the file Infectors which attach themselves to ordinary program files. These usually infect arbitrary .COM and/or .EXE programs, though some can infect any program for which execution is requested, such as .SYS,.OVL,.PRG,amp。.MNU files. File infectors can be either direct action or resident. A directaction virus selects one or more other programs to infect each other time the program which contains it is executed ,and thereafter infects other programs when “they” are executed (as in the case of the Jerusalem) or when certain other conditions are fulfilled. The Vienna is an example of a directaction virus. Most other viruses are resident. The second class is system or bootrecord infectors: those viruses, which infect executable code, found in certain system areas on a disk that are not ordinary files. On DOS systems, there are ordinary bootsector viruses, which infect only the DOS boot 3 sector on diskettes. Examples include Brain, Stoned, Empire, Azusa, and Michelangelo. Such viruses are always resident viruses. Finally, a few viruses are able to infect both (the Tequila virus is one example). There are often called “multipartite” viruses, though there has been criticism of this name。 only the directory entry is. Some consider these infectors to be a third category of viruses, while others consider them to be a subcategory of the file infectors. What are macro viruses? Many applications provide the functionality to create macros. A macro is a series of mands to perform some applicationspecific task. Macros are designed to make life easier, for example, to perform some everyday tasks like textformatting or spreadsheet calculations. Macros can be saved as a series of keystrokes (the application record what keys you press)。 and their advanced macro languages are as plex as general purpose programming languages. When the macro language allows files to be modified, it bees possible to create macros that copy themselves from one file to another. Such selfreplicating macros are called macro viruses. Most macro viruses run under Word for Windows. Since this is a very popular word processor, it provides an effective means for viruses to spread. Most macro viruses are written using the macro language WordBasic. WordBasic is based on the good old BASIC programming language. However, it has many (hundreds of) extensions (for example, to deal with documents: edit, replace string, obtain the name of the current document, open new window, move cursor, etc.). What is a Trojan horse program? A type of program that is often confused with viruses is a ?Trojan horse? program. 4 This is not a virus, but simply a program (often harmful) that pretends to be something else. For example, you might download what you think is a new game。 you have to execute the code in the file to trigger it. This could mean running a program file, or opening a Word/Excel document in a program (such as Word or Excel) that can execute any macros in the document. What kind of files can spread viruses? Viruses have the potential to infect any type of executable code, not just the files that are monly called “program files”. For example, some viruses infect executable code in the boot sector of floppy disk or in system areas of hard drives. Another type of virus, known as a “macro” virus, can infect word processing and spreadsheet documents that use macros. And it?s possible for HTML documents containing JavaScript or other types of executable code to spread viruses or o