Spreading Viruses
Viruses spread most commonly from an infected floppy, email attachment or downloaded file from the Internet, though it is possible for viruses to infect networked computers and be hidden in commercial software
What happens …
The virus arrives at a computer as a piece of program text such as a macro in an attached document or in the “boot sector” code of a floppy, etc. The virus is “latent” until the computer runs the code. When the code is run the virus replicates itself, e.g. sets itself up to become part of other attachments, and possibly does something harmful
Note, a virus that says its doing something harmful could be joking