For decades, emergency rooms in northern Australia struggle to treat a medical enigma. During the summer, swimmers and divers from around the world experience mysterious symptoms after exploring the Great Barrier Reef. Sweating, vomiting, muscle cramps and pain beyond measure… sometimes even death. Yet on their bodies there is hardly a scratch. Some doctors believe it's a heart attack, others drowning. This one-hour special takes a scientific look at the nearly invisible predator, the Killer Jellyfish. How could something so small affect so many lives? In Australia, 2001 and 2002 were the deadliest years on record, but is the worst yet to come?