What's a virus?
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A virus is a program designed to corrupt (and copy itself into) other programs and alter the way they work. The impact of viruses can range from making your computer crash during certain operations, to deleting important files, possibly rendering your computer inoperable.
A special type of virus, which is actually called a "Trojan Horse," is a program designed to look like it's doing something normal while actually playing havoc with your computer's files. However, unlike a true virus, a Trojan Horse can't reproduce itself, nor can it "infect" programs.
Viruses are programs that can only infect other programs, as well as files containing "macros" (a series of commands grouped together to automate a complex series of tasks) such as Microsoft Word and Excel files. Computer viruses can not infect plain text files, such as e-mail messages, Web pages and cookies, nor can they infect pictures or chat groups. (Viruses can be transmitted via e-mail attachments, as discussed in How do viruses get into my computer?)