Gerrard Gyngell asked if it's proper, when forwarding a joke to friends, to delete the sender's information, and also if he should Bcc the recipients.
It's often a good idea, if you're sending anything to a lot of people, to put their addresses in your email program's Bcc box rather than the To or Cc box. That way, only you can see everyone you sent it to; the recipients will only see your address.
Here's why: What you meant as a one-shot joke could easily become an online argument between three people, with an additional twenty or so getting the controversy, unwanted, in their inboxes. People who appreciate the joke won't necessarily appreciate responses from others who thought it was offensive, unfunny, or a waste of time.
There are exceptions, of course. If you know everyone on the list very well, you may want to encourage a multi-person dialog--although generally not about a joke. That happens a lot in my family, where one person posts a political comment that others disagree with, starting an online debate. These seldom last long enough to become annoying.
About removing the forwarded address: This certainly makes the mail less cluttered, and may protect the original sender's privacy. On the other hand, if the person who sent you the joke actually wrote it, removing their name deprives them of deserved credit. As someone who used to write humor professionally, I know what it's like to have my bylined work go viral without my name attached.