Viruses can be written into almost any type of file, so it’s important to be aware of this when you add software to your system. There are known instances of viruses being accidentally included in licensed, shrink-wrapped software, but generally you are safe when installing legally purchased software that you’ve obtained through normal channels.
The two main ways viruses enter your system are through files added to your system from floppy disks (or other removable media like Zip disks) and from downloading from the Internet. You can also get a virus through an e-mail attachment, but not from a plain text e-mail message alone.
A common myth regarding viruses is that they can only be passed into your system through executable program files, or files that are actually programs, not just data. You’d also think, then, that infection couldn’t take place unless the program holding the virus is launched.
With the advent of “macro” viruses, though, this distinction is getting blurred. Macro viruses can exist inside any document whose application uses a macro language, such as Melissa, which was passed in Microsoft Word documents.
In this case, a user can have a clean version of Microsoft Word and simply open an infected Word document, which will then infect the application.