This study examined beliefs in supernatural causation of both Igbo and Yoruba ethnic groups in Nigeria. The study focuses on the beliefs that treatment of any patient involves exorcism of the invading spirit. A significant number of members of the two ethnic groups believe in the active presence of the spirit world, ancestral spirits, gods, and in reincarnation. Consequently, any illness or misfortune is often attributed to the wrath of the gods or neglect of the spirit world. Content analysis of data gathered through participant observation was the primary means used in the analysis of this study. Subjects were Igbo and Yoruba natives of three age groups. These groups included traditional age, transitional age, and modern age generations. This research found that both Igbo and Yoruba ethnic groups tend to turn to beliefs in supernatural causation during serious illnesses, accidents and deaths. However, as individuals move from traditional age to modern age, they tend to believe less in supernatural causation in Igbo and Yoruba communities, despite the introduction of modern medicine. Specific reasons for the persistence of beliefs in supernatural causation in Igbo and Yoruba communities, despite the introduction of modern medicine, is a topic for further research.