Somalis have modified Islam, for example with reference to the social significance of baraka/Ilaah Bari . Baraka is considered a gift from God to the founders and heads of Sufi orders. It is likewise associated with secular leaders and their clan genealogies. A leader has power to bless, but his baraka may have potentially dangerous side effects. His curse is greatly feared, and his power may harm others. When a clan leader visits the leader of another clan, the host's relative receives him first to draw off some of the visitor's power so that his own chief may not be injured. The traditional learning of a wadad includes a form of folk astronomy based on stellar movements and related to seasonal changes. Its primary objective is to signal the times for migration, but it may also be used to set the dates of rituals that are specifically Somali. This folk knowledge is also used in ritual methods of healing and averting misfortune, as well as for divination. Wadaddo help avert misfortune by making protective amulets and charms that transmit some of their baraka to others, or by adding the Quran's baraka to the amulet through a written passage. The baraka of a saint may be obtained in the form of an object that has touched or been placed near his tomb. Although wadaddo may use their power to curse as a sanction, misfortune generally is not attributed to curses or witchcraft. Somalis have accepted the orthodox Muslim view that a man's conduct will be judged in an afterlife. However, a person who commits an antisocial act, such as patricide, is thought possessed of supernatural evil powers. Despite formal Islam's uncompromising monotheism, Muslims everywhere believe in the existence of mortal spirits (jinn), said to be descended from Iblis, a spirit fallen from heaven. Most Somalis consider all spirits to be evil but some believe there are benevolent spirits. Certain kinds of illness, including tuberculosis and pneumonia, or symptoms such as sneezing, coughing, vomiting, and loss of consciousness, are believed to result from spirit possession, namely, the wadaddo of the spirit world. The condition is treated by a human wadad, preferably one who has himself recovered from the sickness. He reads portions of the Quran over the patient and bathes him with perfume, which in Somalia is associated with religious celebrations. In the case of possession by the zar, a spirit, the ceremony of exorcism used to treat it is sometimes referred to as the "zar cult." The victims are women with grievances against their husbands. The symptoms are extreme forms of hysteria and fainting fits. The zar exorcism ritual is conducted by a woman who has had the affliction and thus supposedly has some authority over the spirit. The ritual consists of a special dance in which the victim tends to reproduce the symptoms and fall into a trance. The "illness" enables a disgruntled wife to express her hostility without actually quarreling with her husband. A third kind of spirit possession is known as gelid (entering), in which the spirit of an injured person troubles the offender. A jilted girl, for example, cannot openly complain if a promise of marriage arranged by the respective families has been broken. Her spirit, however, entering the young man who was supposed to marry her and stating the grievance, causes him to fall ill. The exorcism consists of readings from the Quran and commands from a wadad that the spirit leave the afflicted person. Gelid is also thought to be caused by the curse or evil power of a helpless person who has been injured. The underlying notion is that those who are weak in worldly matters are mystically endowed. Such persons are supposed to be under the special protection of God, and kind acts toward them bring religious merit, whereas unkind acts bring punishment. The evil eye, too, is associated with unfortunates, especially women. Thus, members of the Yibir, the numerically smallest and weakest of the special occupation groups and traditionally the lowliest socially, are the most feared for their supernatural powers. Somalis also engage in rituals that derive from pre-Islamic practices and in some cases resemble those of other Eastern Cushitic-speaking peoples. Perhaps the most important of these rituals are the annual celebrations of the clan ancestor among northern Somalis - an expression of their solidarity - and the collective rainmaking ritual (roobdoon) performed by sedentary groups in the south.