The Chironex species of Box Jellyfish in potentially lethal whether it be in Thailand, Philippines, Indonesia, Malaysia or Australia.
In Australia Chironex fleckeri has a fearsome reputation and is responsible for deaths and serious stings all across the tropical northern coast. Its presence, unlike that of its Asian cousins, is seasonal though it is often sighted outside of the October to May range.
This blog with associated home video shows a 7 year old boy Osborne Kewe from Townsville in Australia who received a life-threatening box jellyfish sting. He is alive today only because a nurse on hand kept him alive with CPR while others knew to splash vinegar on his stings and NOT to remove the tentacles by scraping or rubbing or pulling which makes matters worse.