Tiny, venomous and virtually invisible to the human eye, they inflict a sting so painful that if you don't die you may wish you had.
The Irukandji jellyfish, whose stings have hospitalised four swimmers off Queensland's Fraser Coast since Christmas, are on a southern invasion to warner waters.
A woman stung while wearing a full body stinger suit off Cairns in November nearly died from the attack.
Scientists predict the jellyfish, of which there are at least eight species, will reach the Sunshine Coast within the next two decades.
"I cannot begin to explain how excruciating the pain is," jellyfish toxicologist, Professor Jamie Seymour of James Cook University, told news.com.au.
"I've been stung 11 times and each time I've ended up in hospital.
"It's very mild to start with, then it takes about 15 to 20 minutes to kick in and it's overall mind-numbing horrific pain.
"The last time I got stung across the top of my lip, then I got pins and needles in my feet, pain like red hot pokers in my joints and then overwhelming racking body pain and throwing up for 18 hours."
Prof Seymour said the jellyfish, which is square-shaped, translucent and about the size of a thumbnail lives in tropical waters around the world but has been moving south from the Northern Territory and northern parts of Queensland and Western Australia.
"Fifty years ago it was found off Cairns and Townsville, 40 years ago off the Whitsundays and 10 years ago off Fraser Island," he said.
"It is cold-blooded and therefore likes warm waters and is coming further and further south and as water temperatures increase will reach the Sunshine Coast in my lifetime."
Prof Seymour said Cairns doctor Jack Barnes discovered the jellyfish in 1964 and named it after the Aboriginal tribe which identified a sickness with swimming in the ocean at a certain time of year.
Back then the Irukandji only appeared for one month each year.
"Now it's six months. During the season, lifeguards on the beaches drag nets through the water and if they find one Irukandji or a 'smack' - the collective noun - of the jellyfish, the beaches are shut.
"It's only going to get worse. This will [eventually] happen on the Sunshine Coast.
"But we have gone to the tourist industry and the Queensland Government and the Australian Government for funding to research, but no-one wants to fund us."
There is no anti-venom in existence for the Irukandji and compounding the problem Prof Seymour says is the eight different types of the jellyfish have different venom and the venom in the juveniles of the creatures sets off a different physiological reaction to that of the adults.
"In Queensland around 200 people each year are hospitalised from Irukandji stings. When you compare that with 10 to 15 shark attacks in the last 20 years, you can see the need.
"This is an animal we know very little about, how long it lives, its venom, how many offspring it has."
Ayllie White, 39, suffered heart failure after she was stung off Fitzroy Island near Cairns in November while snorkelling with her husband.
She felt something brush her neck and a tingling sensation, and in five minutes a searing pain following by tightness across her chest and difficulty in breathing.
She spent two days in intensive care at Cairns Hospital.