What you believe to be just a data file may include some legitimate executable code, and this code could be infected. For example, a Microsoft Word document contains only word processing data, so it could not be infected. Similarly, a simple e-mail message is just data, but an email message that includes a Word template file could carry a virus in that file. Don't your Web browser or e-mail application to launch Word automatically when it encounters a document. Open the application first, then read the file.
Some viruses can escape detection by antivirus software.
While perhaps not visible to the naked eye of someone viewing a directory, all known virus programs have a signature that can be differentiated from legitimate code. Backups are useless if you back up the virus along with the data. You will have to remove the infected files, and you probably will have to reinstall computer applications from your original software source. This can be time consuming and tedious, but it is possible. Shareware, public bulletin boards and the Internet are the source of most viruses.