Sometimes these urban legends are referred to as FOAF stories: Friend Of A Friend. With that in mind:

COMPUTER VIRUS WARNING...

I just received word from a friend that there is a new computer virus going around called _____. He had the news forwarded from his friend. Therefore, you know that the story is true because it comes from a friend of a friend. Forward this warning to everybody you know. Since you were told by a friend, your friends will have received word of it from a FOAF. Then they can forward it to their friends.

So you see, internet e-mail is simply the newest means of transmission for the oldest categories of tales.