The so-called butterfly fever is a disease native to the isle of Naath. It is believed to be transmitted by the local butterflies, particularly a large black-and-white variety with wings as big as a man's hand, according to Archmaester Ebrose. Even though the Naathi themselves are immune to this sickness, all outsiders who remain too long on Naath fall prey to it.
Fever is the first sign of the disease, followed by painful spasms that make the victims seem to be dancing wildly and uncontrollably. In the last stage, those afflicted sweat blood, and their flesh sloughs from their bones.
Ghiscari seize the islands in the times of the Old Empire thrice. The Valyrians erected a fort of fused dragonstone whose walls survive to the present, a company of Volantene adventurers once built a trade town with timber palisades and slave pens, corsairs of the Basilisk Isles have landed on Naath countless times. All of these invaders fell to the plague. Likewise, when Nymeria's Rhoynar landed on the island, they too fell to the disease, even though they had been welcome with open arms by the Naathi.
However, corsairs long ago learned that the chances of dying of butterfly fever are low as long as they do not remain on the islands for more than a few hours, and lower still if they land at night, as the butterflies that carry it are day creatures. Thus, slavers often attack Naath during the dark of night, forcing the Naathi inland.