The marine creature is known as one of the most dangerous species in the Mediterranean Sea, but its stings are rarely deadly, although excruciatingly painful.
A woman has been placed in an intensive care unit in Sicily after losing consciousness due to what was most likely a sting by a Portuguese man o' war, a marine protozoan known for its long, venomous tendrils.
The head doctor of Catania's San Marco hospital said the woman suffered from exhaustion, breathing difficulties and severe cardiac arrhythmia -- all symptoms caused by the dangerous marine creature.
Quoted by Italy's La Repubblica newspaper, Benedetta Stancanelli said the "lesions on the back and on the legs suggest that this may really be a sting by a Portuguese man o' war".
The incident occurred as the victim was swimming off the Cyclopean Isles, a few kilometers off mainland Sicily.
The woman, who reportedly had underlining conditions, was rushed to intensive care a few hours after having fainted following the malicious encounter.
What is the Portuguese man o' war?
Named after the 18th-century Portuguese warships, the Portuguese man o' war -- or just man-of-war (Physalia physalis) -- is a venomous sea predator classified as "dangerous" in the World Health Organisation's guidelines for safe recreational water environments.
Although it is rarely deadly -- with some three deaths that have been attributed to the sea creature -- its sting is excruciatingly painful and toxic.
Its tentacles can reach up to 30 meters and cause symptoms ranging from "local skin necrosis to neurological and cardiorespiratory problems" leading to death, researchers say.
Although it closely resembles a jellyfish, it is not. It is a "colony of several small individual organisms with specialised jobs". It doesn't move by swimming but uses winds and currents instead.
How widespread is it in the Mediterranean?
Experts say that the number of sightings in the Mediterranean Sea appear to have grown in recent years, sometimes dramatically. But the increase in sightings may also be due to advancements in the field of research, they believe.
In 2010, a woman in Sardinia died after an anaphylactic shock believed to have been caused by its tentacles. Again, the woman appeared to suffer from underlining conditions.
What to do if you're stung by it?
Soak the affected areas in hot water (42˚C), Australian authorities suggest, and apply cold packs and a pain-relieving cream. Lesions must not be treated with vinegar -- and tentacles should only be removed with tweezers or a gloved hand.