Viruses are programs that tell your computer to do something that you don't want it to do. In order for a virus to have any effect, the program must be executed. Some ways of viewing a file can't possibly execute anything, so those are safe. The name of a computer file has an extension at the end. It is usually three letters after a period. That extension tells the computer what program to use to open the file and indicates what kind of information is in the file.

I think of four categories of files. These lists of file extensions (see caution about file extensions) are not exhaustive, but include the most common ones.

1. Quite safe**. (The programs that automatically open these don't run macros.)
   Email message with no attachments
   Written information: extensions .txt, .dat
2. Pretty safe (The programs that run these are more powerful, but have very powerful safeguards. For that reason, I've never heard of a virus making use of these file formats. See caution.)
   Written information: extension .rtf (Why I am pretty sure that it's safe.)
   Web page formats: .pdf, .htm, .html (Why I am pretty sure that these are safe.)
   Audio information: extensions .wav, .aud, .midi (Why I am pretty sure that these are safe.)
   Graphical information: extensions .jpg, .gif, .pic, .bmp (Why I am pretty sure that these are safe.)
   Video information: .mov, .mpeg, .avi (Why I am pretty sure that these are safe.)
3. Potentially dangerous (Supports macros and I have heard of viruses distributed in file types that support macros.)
   Written information: extensions .doc, .wpd
   Spreadsheets, databases, etc: .xls, .mdb, and others
   Screensavers: .scr
4. Most dangerous (Program files)
   programs: extensions .exe, .com, .bat
   scripting: .extensions js, .vbs

 A special category is "zipped" files. (extension .zip) This is just a form of file compression and it's not until you "unzip" the zipped file that you'll know what is in it. The separate files that result when you unzip it will have various extension names and you should treat them as described above.

*Extra caution 1: Sometimes the person who writes a virus tries to fool people with the file name. The ILOVEYOU virus had an attachment called LOVE-LETTER-FOR-YOU.TXT.vbs. Clicking on that attachment ran the virus. Notice that the name of the attachment tried to indicate to people that it was a relatively innocuous text file, but the actual extension was .vbs, which is a quite dangerous file type.

**Extra caution 2:
The solution: Simply save the attachment and don't just double click on it to open it. Instead, open your text editor and, from it, open the file.
The problem: I have just learned <http://www.winmag.com/columns/powerw2k/2000/22.htm> that there are a few file types that are particularly dangerous because they always hide the extension, even if you have set your computer to show extensions. Thus, a malicious person could name the file filename.txt, so that the filename itself looks like it is a text file. If you click on it, your computer recognizes the hidden extension and opens an appropriate program for that extension, not your text editor. If that other program can run programs (or macros or scripts), then it can execute a file that is a virus. The solution, as indicated above, is simply to never double click on a file to open it, but to open it from within the appropriate type of program.

Here is another article on hidden file extensions. Probably it will tell you more than you want to know! <http://www.winmag.com/columns/optwin/2000/01.htm>

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Last updated January 21, 2001. Mary Parker