A computer virus is a small program which attaches itself to a file such as an image or an application for example. An email virus is a computer virus which spreads by means of emails. A plain text email cannot possibly contain a virus, however images conveyed with rich mail clients, and attachments in general, are vulnerable for virus propagation.
To avoid email viruses you need to have a capable anti-virus program installed; I can really recommend both Webroot AntiVirus with AntiSpyware 6.1 and Norton AntiVirus 2009. In any case make sure your antivirus program is regularly updated so as not to get infected by an email virus which has just evolved in the wild.
When you receive an email containing an attachment use common sense before opening it. Do you know the sender, or do you trust the sender? Consider scanning the attachment with your anti-virus program instead of opening it.
If the email contains links to the Web think a moment before clicking it. Some Web sites are capable of passing a virus to your computer just by visiting their site (a so called drive-by download). A linking scanning program such as Web of Trust (WOT) can help you tell the good sites from the bad.
Should you receive an email or an attachment which sets off an alert by your antivirus software then it is recommend to run a full anti-virus scan of your computer. This can usually take place in the background and does not interfere with your normal use of the computer.
Most of the times the sender of a legitimate email is not aware that he or she sends infected mails, so if you happen to know the sender you might want to let them know should you receive an email which contains a virus.
However, beware of hoaxes. A hoax is bad joke asking the user to carry out an action on a safe system, such as for instance deleting s system file, which will damage the computer. Please, do not forward them to all people in your address book. Instead talk with the computer administrator.
There is no absolute protection against getting infected by an email virus. However, you should do everything possible to avoid email viruses including self-propagating virus called computer worms. What makes them so nasty is that worms try to send their payload to everybody of your contact list. This can severely undermine your reputation.