The origin of fever or jwara in Hindu tradition is traced to the infamous Daksha Yajna in which Sati self immolated and Shiva wreaked havoc. Jwara sometimes talked as a demon is believed to have appeared from the perspiration that fell from the forehead of Shiva.
Puranas states that Shiva was deliberately not invited by Daksha, father of Sati, for the Yajna he conducted. Sati who went to ask about this insult of her husband was insulted and jeered. Shiva had forewarned Sati that she will be insulted at the Yajna. Unable to bear this unruly behavior, She self immolated.
Enraged at the death of Sati, Shiva destroyed the yajna. Shiva Ganas destroyed all that they saw. The Yajna took the form of a deer and flew into the sky. Seeing this Shiva took his bow and took aim at the deer. In this act, Shiva got furious and a fierce looking perspiration fell on to the earth.
As the drop fell on earth a terrible fire was produced. From it appeared a short and ugly looking demon – he had red eyes, golden tawny beard, hair on the head was standing erect, looked around like an owl and wore red clothes. This was Demon Fever or Jwara.
He started to inflict all that was around him. Brahma requested Shiva to minimize the effects of Jwara. He then distributed it on various living beings. In humans, it is present from birth till death.