Pazuzu is an Assyrian/Babylonian demonic god who was most popular in the first millenium BCE. She was the daughter of Hanbi (also Hanba), king of the demons of the underworld, and brother to Humbaba, the demon-god protector of the Cedar Forest in The Epic of Gilgamesh who is killed by the heroes.
She was the demon of the underworld (where all demons were thought to reside) in control of the west and south-west winds which brought famine during the dry season and, in the rainy season, tearing storms and locusts. As she was the force behind the destructive winds and their threat, she was also considered the best defense against them. Similar in this way to the Egyptian god Set, prayers to Pazuzu were intended to divert her natural inclination toward destruction to the more benevolent ends of protection. Since it was clear she had great power to harm it was thought he was equally potent in protecting one from the very danger he presented.
Even so, although she was never considered the most benevolent of supernatural beings in ancient Mesopotamia - and was certainly regarded as an evil demon - she was not evil incarnate and was frequently invoked for protection from evil. She was particularly powerful in protecting pregnant women and children from the demon-goddess Lamashtu who preyed on unborn and newly-born babies.
The term "demon" in the modern day always carries with it the connotation of evil but this was not so in the ancient world. The English word "demon" is a translation of the Greek word daimon which simply meant "spirit". A daimon could be good or evil, depending on its intentions and the results of a visitation. In ancient Mesopotamia, as in other cultures of the ancient world, demons were often sent by the gods as punishment for sin or to remind one of one's duty to the gods and others in one's community. Demons were not always evil and even those who were, like Pazuzu, were still capable of good deeds.
In the Akkadian/Babylonian myth of the Atrahasis, human beings have grown too numerous, too loud, and far too fertile. Further, they live so long that more of them are being born at any given time than those who die. They cover the earth and disturb the god Enlil with their noise to such an extent that he decides to destroy them in a great flood. After the flood waters subside, the god of wisdom Enki proposes a plan to re-populate the earth: the gods will create a new kind of human with a shorter life span and greater threats to their lives on a daily basis. Henceforth there will be disease, miscarriages, impotency, sterility, attacks by wild animals, and all other manner of death waiting in a day.