The Irukandji jellyfish, considered by researchers to be one of the world's most poisonous creatures, is only the size of a thimble.
A tiny jellyfish that is almost invisible, the Irukandji has a painless sting, but can cause massive pain and suffering afterward.
The threat is not just for swimmers. It's been reported that a 45-year-old Filipino was stung by an Irukandji while fishing from a bulk carrier.
The man, who was airlifted to hospital, was 25 meters above sea level when stung. It's thought that the creature was blown up with sea spray. Jellyfish nets aren't effective either because this tiny animal can slip through.
Wearing a "stinger suit" is advised when swimming in Queensland or the surrounding region during Irukandji season, and never swim on beaches without lifeguards with reliable local knowledge.
This little beast is most problematic around the east coast of Australia.
5. Wandering spiders
One of the most venomous and dangerous spiders in the world is the Brazilian "wandering spider," or "banana spider," although its bites are also famous for another nasty side effect.
The venom can cause involuntary and long-lasting priapism for men.
Chances are the victim won't be able to put it to good use before he dies from the other components of this potent toxin.
The very name "bloodworm" sounds macabre. But the damage they can do to the human body, and the sheer scale of the problems caused by these tiny parasites, is far scarier.
The larvae (cercaria) of these little animals live in freshwater snails, usually in relatively clear water. But when they sense a warm-blooded host, such as a human being swimming past, they swim towards it, penetrate the skin, move around the body, and finally make their home in the intestines, or the bladder, where they can live and reproduce for up to 12 years gradually destroying the organ that they made their home.
It sounds like something out of a sci-fi film, but it is definitely not rare. Up to 200 million people suffer from schistosomiasis (bilharzia) worldwide and a large proportion of these will die prematurely from the illness.
The solution -- don't swim in slow moving fresh water anywhere in sub-Saharan Africa or South America. In other places check first. People have even contracted the disease in un-chlorinated swimming pools in Rio de Janeiro.
3. Sand flies
In South America a sand fly bite can lead to the eyes or nose being eaten away by bacteria, and in the Indian sub-continent the disease that they carry can (and often does) kill.
The tiny flies don't make a buzzing noise and can sneak through mosquito nets, making any form of defense difficult.
Insect repellent, long-sleeved clothing and avoiding sand fly habitats are the best tactics.
2. Day-biting mosquitoes
Researchers are working on a dengue fever vaccine. For now, the only protective measure is to cover up and wear repellent.
Most of us know that we should avoid going out at night without some form of protection against evening-biting mosquitoes, whether that is in the shape of long-sleeved clothes, insect repellent or simply staying inside.
However, day biting mosquitoes can spread a disease called dengue fever, otherwise known as "break-bone fever."
The first time the victim contracts it, he or she will probably survive. The second time it's possible to develop dengue hemorrhagic fever, in which case one's organs can turn to mush and bleeding proceeds from every orifice -- and there's a good chance of death.