One story says Goddess Durga has incarnated as little Katyayani, the daughter of sage Katyayan to destroy all arrogant evil demonic forces of the world, in her real form as Durga, she killed many demons that were sent by Kaalkeya.
A demon named Jwarasur, the demon of fever, started spreading incurable diseases to Katyayani's childhood friends, such as cholera, dysentery, measles, smallpox etc. Katyayani cured the diseases of some of her friends.