A (meme?) virus which is especially pernicious.
Perhaps it's because communication via e-mail is still relatively new to a lot of people, but a lot of people will receive an e-mail detailing alleged facts and, simply by dint of receving it via e-mail, will assume the contents describe the truth. The gullibility virus is then communicated to every person that receives a forwarded copy of the mail.
E-mails such as these aren't really harmful other than the amount of time spent by people at the CDC or local law enforcement agencies debunking whatever alarming contents the e-mail detailed. However, you can help stamp out the gullibility virus. Simply do a search for urban legend or net hoax and you will find numerous resources to help you fight this especially wicked virus. A good place to start is http://urbanlegends.about.com.
And stop sending postcards to that little boy with cancer. He died nearly ten years ago, and his family has to be reminded of that painful fact every day because they're still being deluged with postcards from clueless well-meaning dupes!