The causes of disease were thought to be
- Supernatural theory of disease
- The theory of humors
- The concept of contagion
- Miasmatic theory of disease
- The theory of spontaneous generation etc.
Discoveries in microbiology and the investigations of Louis Pasteur marked a turning point in the etiological concepts of diseases.
Causation of Disease:
- Germ theory of disease
- Epidemidogical Triad
- Multifactorial causation
- Web of causation.
Germ Theory of disease
Emphasis has shifted from empirical causes like air to microorganisms as sole cause of disease. The concept of cause in this theory is referred to as one to one relationship between causal agent and disease.
The disease model is
Modern medicine does not stick to germ theory of disease because now it is well established that a disease is not caused by a single agent but depends upon a number of factors which contribute to its occurrence.
Limitations of Germ Theory
Not every individual exposed to agent develops the disease. There are other factors that determine the occurrence of disease. Tuberculosis ------- Not everyone exposed to tuberculosis bacteria develop the disease.
It is not the agent alone which causes the disease. There are other factors relating to the host and environment which are equally important to determine whether or not disease will occur in the exposed host. As a result an epidemiological triad came into existence where in agent, host and environment - all three are involved.
This helps epidemiologists to concentrate on different class of factors especially with regard to infectious diseases.
Multi factorial Causation
The concept of disease is not a single factor but due to multiple factors.
As a result of advances in public health, chemotherapy, antibiotics and vector control, communicable diseases are reduced. This was replaced by many modern diseases namely lung cancer, coronary heart disease, mental illness, diabetes etc. which are not caused by any germs or can be cured or prevented by the traditional method of isolation, immunization or improvements in sanitation. This lead to the realization that single cause idea was an over simplification and there are other factors causing the disease viz.,
- Genetic and
E.g. Tuberculosis occurs not only due to the contact with the agent tubercle bacilli but also due to other factors which includes poverty, overcrowding and malnutrition. Similarly the diseases like coronary heart disease, cancer and diabetes occur due to excess consumption of fat, smoking, alcoholism, physical inactivity and obesity. Many of these are life style factors and human behavior can be called as diseases of affluence. The epidemiology is a multifactorial causation. Multifactorial causation offers multiple approaches for the prevention or control of disease.
This model of causation of disease considers all the predisposing factors and their interrelationship with each other. It is more suited to chronic diseases where agent is not known.
The web of causation does not imply that the disease cannot be controlled unless all the multiple causes or factors are controlled or removed. Sometimes removal or control of one link key factor in the pathogenesis is enough to control the disease. The relative importance of these factors can be expressed in terms of ‘relative risk’.
Relative risk is the ratio between the incidence of disease among exposed persons and among non exposed.