a particular microbe was to be considered the cause of a particular human illness. The concept, in general, served well. It provided the means for controlling many infectious and chemically-induced illnesses. Unfortunately the very success of the germ theory encouraged a belief in a simplistic chain of causation one specific organism giving rise to one (more or less) specific disease. As the list of factors known to be capable of inducing human disease has lengthened (Howe 1976a), it has become clear that a particular disease manifestation may have more than one causal antecedent. Exposure to a known cause of illness does not always lead to the expression of that illness. Either way response is conditioned by the genetic make-up of the individual. Furthermore the identification of a causal antecedent does not necessarily provide the ability to prevent or control the ailment. So it is now known that the one cause one disease model is far too simple. Illness in an individual is the result of a multitude of prior circumstances; causal circumstances differ from one individual to another, even when the manifestations of their illness are indistinguishable. Minor differences in diet or in physical or chemical environment determine the reaction of a person to a given microbial or genetic stimulus, and vice versa. Indeed the causal antecedents of illness in any individual comprise a web of intertwined circumstances that in their full breadth and complexity lie quite beyond one's understanding. Fortunately, to develop effective preventive measures it is not necessary to understand the entire causal web or multifactorial complex but merely to identify significant strains (provocative factors), the disruption of which can lead to alteration in the whole structure, that is, to preventive action (Kotin 1972). The World Health Organization (W.H.O. 1965) defines health as 'a state of complete physical, mental and social well-being and not merely the absence of disease or infirmity.' Health is not an absolute quantity but a concept whose standards are continually changing in different lands with the acquisition of knowledge and the establishment of cultural objectives. To be truly healthy a person should enjoy a balanced relationship of the body and mind and complete adjustment to the total environment. It is a state of mind as well as a condition of the body it represents ecological balance, harmony in the environment. Disease, on the other hand, is maladjustment in the environment, the response being conditioned by the genetic make-up of the individual. The concept of 'maladjustment in an environment' or 'response to adverse stimuli in the environment' dispenses with the 'one germ-one disease' hypothesis or the uni-causal explanation of disease. Rather does it invoke a range of disease-producing factors or hazards (May 1974). In the United Kingdom communicable diseases, such as the measles or chicken pox most of us experienced as children, are primarily a thing of the past. They have been