A virus is an executable program. It is usually hidden in one of those cute little dancing baby type programs. Friends often send viruses to friends unknowingly in cute things they got from other friends. Best thing to do is trash ALL attachments unless they are information you have specifically asked someone to send you that you need to do your job. Word has macros that can be placed in documents.
Macros are executable programs.
You do not know what they will do, so never run them. You can set word to never run macros without permission. Everything you need to know about viruses. Lots of info about viruses and hoaxes. Computer Virus Myths - Very interesting truths and falsehoods about all those email messages you get from folks.