A component that appears in every pie or pathway is called a necessary cause, because without it, disease does not occur. A disease may have more than one sufficient cause, with each sufficient cause being composed of several component causes that may or may not overlap. The complete pie, which might be considered a causal pathway, is called a sufficient cause. The individual factors are called component causes. After all the pieces of a pie fall into place, the pie is complete - and disease occurs. An individual factor that contributes to cause disease is shown as a piece of a pie. One such model was proposed by Rothman in 1976, and has come to be known as the Causal Pies.( 42) This model is illustrated in Figure 1.17. Component causes and causal piesīecause the agent-host-environment model did not work well for many non-infectious diseases, several other models that attempt to account for the multifactorial nature of causation have been proposed. Environmental factors include physical factors such as geology and climate, biologic factors such as insects that transmit the agent, and socioeconomic factors such as crowding, sanitation, and the availability of health services. Susceptibility and response to an agent are influenced by factors such as genetic composition, nutritional and immunologic status, anatomic structure, presence of disease or medications, and psychological makeup.Įnvironment refers to extrinsic factors that affect the agent and the opportunity for exposure. Opportunities for exposure are often influenced by behaviors such as sexual practices, hygiene, and other personal choices as well as by age and sex. A variety of factors intrinsic to the host, sometimes called risk factors, can influence an individual’s exposure, susceptibility, or response to a causative agent. Host refers to the human who can get the disease. While the epidemiologic triad serves as a useful model for many diseases, it has proven inadequate for cardiovascular disease, cancer, and other diseases that appear to have multiple contributing causes without a single necessary one. These include chemical contaminants (such as the L-tryptophan contaminant responsible for eosinophilia-myalgia syndrome), as well as physical forces (such as repetitive mechanical forces associated with carpal tunnel syndrome). Over time, the concept of agent has been broadened to include chemical and physical causes of disease or injury. A variety of factors influence whether exposure to an organism will result in disease, including the organism’s pathogenicity (ability to cause disease) and dose. Generally, the agent must be present for disease to occur however, presence of that agent alone is not always sufficient to cause disease. Development of appropriate, practical, and effective public health measures to control or prevent disease usually requires assessment of all three components and their interactions.Īgent originally referred to an infectious microorganism or pathogen: a virus, bacterium, parasite, or other microbe. Different diseases require different balances and interactions of these three components. Two ways of depicting this model are shown in Figure 1.16.Īgent, host, and environmental factors interrelate in a variety of complex ways to produce disease. In this model, disease results from the interaction between the agent and the susceptible host in an environment that supports transmission of the agent from a source to that host. The triad consists of an external agent, a susceptible host, and an environment that brings the host and agent together. Among the simplest of these is the epidemiologic triad or triangle, the traditional model for infectious disease. CausationĪ number of models of disease causation have been proposed. As noted earlier, one important use of epidemiology is to identify the factors that place some members at greater risk than others. Section 8: Concepts of Disease OccurrenceĪ critical premise of epidemiology is that disease and other health events do not occur randomly in a population, but are more likely to occur in some members of the population than others because of risk factors that may not be distributed randomly in the population.
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