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A disordered or abnormal condition of an organ or other part of an organism resulting from the effect of genetic or developmental errors, infection, nutritional deficiency, toxicity, or unfavorable environmental factors; illness; sickness is called disease. It might be any harmful condition, as of society.

Disease is an impairment of the normal state of the living animal or plant body or one of its parts that interrupts or modifies the performance of the vital functions, is typically manifested by environmental factors (as malnutrition, industrial hazards, or climate), to specific infective agents (as worms, bacterial, or viruses), to inherent defects of the organism (as genetic anomalies), or to combinations of these factors.


  • The invasion and multiplication of microorganisms such as bacteria, viruses, and parasites that are not normally present within the body is called Infection. An infection may cause no symptoms and be subclinical, or it may cause symptoms and be clinically apparent. An infection may remain localized, or it may spread through the blood or lymphatic vessels to become systemic (body wide). Microorganisms that live naturally in the body are not considered infections. For example, bacteria that normally live within the mouth and intestine are not infections.
  • In general, viral infections are systemic. This means they involve many different parts of the body or more than one body system at the same time; i.e. a runny nose, sinus congestion, cough, body aches etc.
  • They can be local at times as in viral conjunctivitis or “pink eye” and herpes. Only a few viral infections are painful, like herpes. The pain of viral infections is often described as itchy or burning.

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Epidemiologic triangle model of disease causation

  1. Multifactorial causation
  2. Web of causation

Germ theory of disease causation

According to this theory, there is one single specific cause of every disease. This refers to one relationship between the causative agent and disease.

Epidemiologic triangle model of disease causation

A common model used in public health illustrates the relationships among an agent, a host and the environment. It applies to biological, chemical and physical agents. For a disease or injury to occur, the basic elements of disease or injury causation and an adequate chain of transmission (i.e. environmental pathway) must be present. Disease occurs when an outside agent capable of causing the disease or injury meets a host that is vulnerable to the agent in an environment that allows the agent and host to interact. These basic concepts help guide the selection of health strategies to prevent health problems.

Multi factorial causation

Epidemiological theory is not applicable for noninfectious and chronic disease like coronary artery disease etc. because it has many cause or multiple factors. This theory of causation helps to understand the various associated causative factors, priorities and plan preventive and plan measure to control the disease.

Web of causation

According to Macmohan and Pugh disease never depends upon single isolated cause rather it develops from a chain of causation in which each link itself is a result of complex interaction of preceding events these claims of causation which may be the fraction of the whole complex is known as web of causation.

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