The Irukandji jellyfish is a tiny species of jellyfish that lives in the waters of Australia. Although they're usually no more than 1 cubic centimiter in volume, they can be extremely poisonous.
What makes them deadly is that they can fire their stingers into their victim. If you get stung, you develop something called the Irukandji syndrome. If left untreated, this syndrome may cause the victim to go into cardia arrest and die within 20 minutes!
Since they're so tiny, it was originally dificult to determine what caused the syndrome. So in 1964 a (very dedicated) doctor decided to prove that it was the tiny jellyfish's making by capturing and letting himself get stung