If you open a file that has a macro and the macro is, in fact, a virus, you have just infected your computer. One may also attach program files. Greeting cards are often attached to e-mail as executable files or .exe files. A malevolent person could give you a virus program rather than a greeting card. Accepting and executing material from over the Internet has its risks.
New viruses appear every month.
After you create an outgoing message with an appropriate recipient, perhaps the easiest way to attach a file is to left-click on the word Message on the menu bar. From the drop-down menu, choose attachments. The dialog box that appears will allow you to browse through your disk. Select the name of the file you wish to attach and left-click on the Open button. This will return you to the outgoing message and you will see the file name and its path on the Attached line.