Just because an email says that it comes from a specific email address does not mean that it actually came from that email address. More than likely, you received some kind of spam with a virus attachment that appeared to come from your friend, but in actuality, did not come from them. You then sent the email "back" to them (even though it didn't originate from them), unwittingly also sending them the virus.
I can compose an email and make it seem like it's coming from
bobama@whitehouse.gov,
obinladen@alqaeda.org,
jchrist@heaven.net, or any email address I choose. You can, too. Typically, people don't do this, but it's very simple to do. That's likely what happened -- the originator of the email you received modified the email so that it appeared to come from your friend.
It
is a virus, and you
did send it to them, but they should have up-to-date virus protection that prevented the virus from doing any harm. The virus didn't originate from your computer -- it is
impossible for a virus to spontaneously come into existence on a computer -- they have to come from
somewhere, and that somewhere was the "joke" email that was from your friend but really wasn't from your friend.
No need to change your email address... next time you receive something that is questionable from an email address you're familiar with,
do not forward the email back to that person... instead, compose a
new, blank email message to them asking whether they sent it to you or not. When you create a new email message on your Mac, it is impossible for that new email message to contain a virus. By forwarding the original email back to them, though, you passed on the attachment (the virus) to them as well.
Clear as mud?
