There are many types of jellyfish, most of which are harmless to humans. Some jellyfish cause slight skin irritation and pain that generally subsides after about 15-30 minutes.
There are two types of tropical marine jellyfish that can cause serious injury; these are the Irukandji and Box Jellyfish.
Although the box jellyfish is large, it is transparent and almost impossible to see in the water.
The Box Jellyfish causes a severe whip-like, burning red laceration marks to the skin. Severe stings may cause the victim to stop breathing or their heart to stop beating.