They only damage data. There are three common types of computer viruses, and many virus hoaxes. You should be concerned about viruses, but you should also be able to identify a virus hoax. Viruses can be sent with the document and activated when the macro is run. You may not even see any activity to indicate that a virus has infected your computer.
Any diskette can carry a virus.
Always check a strange diskette or a diskette that you have had in another computer for viruses before you use it. Programs on diskette are shared less often than data files on diskette. Even these diskettes are not to be trusted. They want to inform their friends about it, so they e-mail everyone they can think of to warn them. There are no viruses that are started when you open e-mail.