Theories about health and illness deal with the ideas people use to explain how to maintain a healthy state and why they become ill. Ideas about illness causation may include such ideas as breach of taboo, soul loss, germs, upset in the hot-cold balance of the body, or a weakening of the body's immune system. Theories of illness causation derive from the underlying cognitive orientation of a cultural group, and therapeutic practice usually follows the same cultural logic.
Anthropologists often divide theories of illness into two broad categories: personalistic and naturalistic. In a personalistic system, illness is believed to be caused by the intervention of a sensate agent who may be a supernatural being (a deity or dead ancestor) or a human being with special powers (a witch or a sorcerer). The sick person's illness is considered to be a direct result of the malign influence of these agents. In naturalistic causation, illness is explained in impersonal terms. When the body is in balance with the natural environment, a state of health prevails. However, when that balance is disturbed, illness results. Often, people invoke both types of causation in explaining an episode of illness, and treatment may entail two corresponding types of therapy.
According to personalistic theories of illness, illness may be linked to transgressions of a moral and spiritual nature. If someone has violated a social norm or breached a religious taboo, he or she may invoke the wrath of a deity, and sickness—as a form of divine punishment—may result. Possession by evil spirits is also thought to be a cause of illness in many cultures. This may be due to inappropriate behavior on the part of the patient—failure to carry out the proper rituals of respect for a dead ancestor, for example—or it may be simply due to bad luck. Sometimes, one person's envy of another's good fortune is believed to exert a malign influence through the "evil eye," which can result in illness or other calamities. Witches and sorcerers are malevolent human beings who manipulate secret rituals and charms to bring calamity upon their enemies. Recovery from an illness arising from personalistic causes usually involves the use of ritual and symbolism, most often by practitioners who are specially trained in these arts.
Naturalistic theories of disease causation tend to view health as a state of harmony between a human being and his or her environment; when this balance is upset, illness will result. The humoral system is a naturalistic approach to illness whose roots are over two thousand years old. Humoral concepts of health and illness are widely found in India, southeast Asia, China, and, in a somewhat different form, in Latin and South America. Maintaining humoral balance involves attention to appropriate diet and activity, including regulating one's diet according to the seasons. Illnesses may be categorized into those due to excess heat and those due to excess cold. Treatment of an illness of overheat would involve measures such as giving cooling foods and application of cool compresses.
In India, the ancient system of Ayurveda is based on naturalistic ideas of illness causation. Therapy in Ayurveda includes a vast pharmacopeia of preparations made from herbs and minerals, and dietary advice also forms part of every prescription. Ayurveda is actively practised in India today and has shaped the way Indians think about their bodies in health and in illness.
An important set of theories about health and illness, often called "vitalist" theories, is widespread in China, South Asia, and Southeast Asia. When vital forces within the body flow in a harmonious pattern, a positive state of health is maintained. Illness results when this smooth flow of energy is disrupted, and therapeutic measures are aimed at restoring a normal flow of energy in the body. In China this vital force is known as "chi"; in India it is called "prana." In China the ancient art of acupuncture is based on this understanding of the body. Acupuncture needles are inserted at various points along the "meridians," or energy orientations, of the body. The stimulation of the needles helps to restore a proper flow of energy within the body. In India, yoga (particularly hatha yoga, the physical form of yoga) is used therapeutically to restore a balanced energy flow through body and mind.
Biomedicine (modern traditional medicine) is founded on a naturalistic set of theories about the body, and these theories are continually evolving. One of the core theories of contemporary biomedicine, the germ theory of disease, is of relatively recent origin. According to an older biomedical concept, the miasma theory of disease,
Alternative therapies (also called complementary therapies) have been rapidly gaining in popularity worldwide. These therapies are diverse, ranging from traditional treatments adapted from their lands of origin, such as acupuncture and shiatsu, to newly developed forms of therapy such as therapeutic touch. Many alternative therapies have underlying theories of illness causation that are quite different from that of biomedicine. In therapies such as acupuncture, reiki, and shiatsu the concept of vital energy, or chi, is the basis for the practice. Successful therapy is that which regulates and harmonizes energy flows. In iridology, particular areas on the iris of the eye are thought to correspond to specific body organs, and a diagnosis of malfunction of the organs can be made by an examination of the iris. Conceptually, this theory of illness links to an ancient philosophical system of "homologies" that makes connections between the cosmic and the terrestrial; between the outer environment and the inner; and between the external body and the internal body.
All theories of health and illness serve to create a context of meaning within which the patient can make sense of his or her bodily experience. A meaningful context for illness usually reflects core cultural values, and allows the patient to bring order to the chaotic world of serious illness and to regain some sense of control in a frightening situation.