A prince once wishing to be blessed with a son, made elaborate sacrifices to propitiate Brahma. But something went amiss in the performance of rituals and instead of a son, a beautiful maiden arose from the sacrificial fire. Brahma named her Shitala, the cooling one. Shitala inquired Brahma about her status in the world. He assured her that humans would always worship her, provided she carried on her, seeds of the urad (black gram), signifying that she embodied the powers of this particular lentil. Shitala then expressed desire for a companion and was directed to Shiva. Impressed by her devotion Shiva agreed to give her a companion.
From the sweat of Shiva's asceticism was born a demon of prodigious size, who was cut down into three pieces by Shiva himself. Brahma put him back together again. But the demon now had three pairs of arms and legs. He was given the name, jvarasura, or the demon of fever. Shiva assigned him to be Shitala's companion.
Shitala required a beast of burden to carry her load of lentils and Jvarasura suggested an ass for the purpose. Shitala then disguised herself as an old woman, and Jvarasura as a young boy. With their bag of lentils on the back of their ass, they visited all the divine beings. The lentils got converted into pox germs and whosoever they visited was afflicted with fever and small pox. Thus affected, the gods asked for her mercy and promised her that they would worship her, provided she went to the earth carrying her packload of germs with her.
Agreeing, Shitala descended to the earth. To display her prowess she first paid a visit to King Birat, an energetic worshipper of Shiva. Birat though acknowledging her status as a goddess, refused to giver her precedence over Shiva. Shitala threatened him by apprising him of her power to inflict small pox, but the king did not budge from his position. Thus incensed Shitala called upon seventy five different types of the pox to her service, which wreaked havoc on the people of King Birat. But the king even then refused the citizens permission to worship her, the outcome of which was widespread epidemic and deaths. But finally the king, realizing his folly, did relent, and was miraculously restored back to health, without any residual blemishes, as were all others who acknowledged her supremacy.
Shitala embodies both the disease and its remedy. She hates dirt, and the householder who wants her to visit his place to cure it of disease, especially small-pox, must first thoroughly clean it, since a dirty house is the birthplace of most diseases. For this purpose the Great Goddess carries in her hands a silver broom. The winnow fan she hods in her upraised right hand is to collect the results of this cleansing operation and to sift the healthy grains from the diseased ones. The small bowl in the other right hand is to collect the rogue germs, which are then to be banished from the house. The clean house is then to be ritually declared as purified by the sprinkling of Ganga jal (water from the river Ganges). She thus also holds a water pot in one of her hands.