also known as Limp Biscuit, Ookie Cookie
With contributions by Anonymous. - Thanks!
While most of the really nasty STDS actually require your mouth and other bits to make contact with the skin and mucus membranes of your partners, it's not a good idea to swallow the sperm of someone who has AIDS, Hepatitis C, etc. The virus won't survive your stomach acid, but any cuts in your mouth or gums could mean transmission.
As you might imagine, this started in Australia. Take one biscuit (cookie in the United States). Take a group of (generally) young men/boys. All participants masturbate until orgasm which each one does on on the biscuit/cookie. Last person to ejaculate eats the cookie completely with all it's contents.