Email messages forwarded multiple times, like jokes or email hoaxes, are an easy way for spammers to harvest email addresses. Protect yourself and your friends by removing all addresses when you forward emails. It's easy: just highlight the addresses showing in the message body, and select Delete.
Another way to protect your friends is to keep their email addresses private. Do this by using the BCC field instead of the To field for all your recipients. It works the same, but the email will show undisclosed recipients instead of your friends' names and email addresses.