A computer Virus, is specific to an operating system. For example, if you receive an email with a virus for Windows Vista attached to it, YOU will not get the virus, as you are not running the Windows OS. If however, you forward that email to someone who does have Windows Vista computer, they can get the virus. If you forward to someone who has Windows XP, they won't get it, as they are not running Windows Vista.
You can forwarded a virus in an email, but not have it yourself. Why do you think that you have a virus?
Running a virus protection software, is about the only way you can safe guard against this.
See these links for some ideas - http://www.macupdate.com/app/mac/15850/clamxav. and http://www.sophos.com/en-us/products/free-tools/sophos-antivirus-for-mac-home-ed ition.aspx. You may also want to look here - http://www.reedcorner.net/mmg/
If that isn't the case, and it's not required that you have a/v software installed, don't.