It is defined as any deviation from normal functioning or state of complete physical or mental well-being. Disease has several spectrum ranging from mild cases to severe illness. The end result (outcome) of a disease may be recovery, disability or death.

Concept of causation

Previously, the ‘germ theory of disease’ was in vogue. Ac­cording to this, diseases were thought to be due to specific germ or agent. Now, the concept is that diseases occur due to multiple factors i.e., agent, host and environment. These three factors for the epidemiological triad. For example tubercule occurs not merely due to the presence of tuberculosis bacilli; (agent). Other factors like poverty, overcrowding, malnutrition etc. (environment) contribute to its occurrence. Also it is known that multiple factors like excess of fat intake, smoking, lack of physical exercise and obesity are involved in the causation of coronary heart disease If these factors are known, diseases can be controlled or prevented by eliminating these factors.



Natural history of disease

Diseases occur due to a complex interaction between host, agent and environment. Natural history of disease is the study of evolution of disease from the earliest stage of prepathogenesis to its termination (in the form recovery, disability or death). Every dis­ease has its own natural history. But it need not be the same in all individuals. Natural history can be established by Cohort studies, Cross-sectional and Retrospective studies.



The natural history of disease consists of two phases:

1. Prepathogenesis – the process in the environment

2. Pathogenesis – the process in man



This is the phase before the onset of disease in man. The disease agent has not yet entered man But the factors needed for its interaction with the human host are already present in the envi­ronment. Potentially, we are all in the prepathogenesis phase of many diseases.


The causative factors of disease are AGENT, HOST and EN­VIRONMENT. These three factors are referred as epidemiological triad. The mere presence of these three factors is not sufficient to initiate a disease. An interaction of all these three factors is neces­sary to initiate the disease process. In prepathogenesis phase, the disease agent is already present but it has not entered man.



This phase begins with the entry of disease agent into man (host). There is a certain interval of time before the onset of clinical signs and symptoms of the disease. This period is called incuba­tion period. During this period the disease agent multiplies and induces tissue and physiological changes.

Incubation period is followed by early prepathogenesis. Dur­ing this period, the signs and symptoms are not clear-cut. This is followed by late pathogenesis when there are clear-cut signs and symptoms. The final outcome of the disease may be recovery, disability or death.

Importance of natural history

Each disease has its own natural history; but it is not necessar­ily the same in all individuals. If the phase of natural history is known, appropriate level of prevention can be applied.



The first link in the chain of disease transmission is the disease agent. The disease agent is defined as ‘a living or non- living substance or the excessive presence or absence of a force which may initiate or perpetuate a disease process’.

Disease agents may be broadly classified as:

1. Biologic agents


2. Nutrient agents

3. Physical agents

4. Chemical agents

5. Mechanical agents

6. Social agents

7. Absence or excess of health factors.

1. Biologic agents: They are living organisms such as bacteria, viruses, protozoa and fungi.

2. Nutrient agents: They are proteins, fats, carbohydrates, vitamins and minerals. Any excess or deficiency of these nutrients can result in diseases.

3. Physical agents: They are exposure to heat, cold, pressure, electricity, radiation etc.

4 Chemical agents: They may be

a) exogenous (present outside) such as fumes, dusts, metals, gases, etc.

b) endogenous (produced in the body) such as urea, uric acid,

ketones etc.

5. Mechanical agents: Various mechanical forces may result in injuries (like crushing, tearing, sprains) and even in death.

6. Social agents: They are poverty, smoking, alcohol, drug abuse, unhealthy life-styles, social isolation etc.

7. Absence or excess of health factors: Excess or lack of hormones,nutrients etc can lead to a variety of diseases.


The human host is supposed to be ‘soil’ and the disease agent as the ‘seed’. The host factors are classified as:

1. Demographic characteristics such as age sex and ethnicity.

2. Biological characteristics such as genetic factors, blood groups, enzymes, immunological factors etc.

3. Social and economic characteristics such as education, occupation, income, housing etc.

4. Life-style factors such as nutrition, exercise, use of alcohol, drug abuse, smoking etc.


The environment in which man lives is an important factor in the causation of diseases. Environment is classified as:

1. Physical environment

2. Biological environment

3. Psychological environment.

1. Physical environment: It refers to non-living things and physical factors such as air, water, light, noise, radiation etc. Alteration of physical environment has produced problems such as air pollution, water pollution, noise pollution, radiation hazards etc. All these pollutions produce a variety of diseases.

2. Biological environment: These are the living things which surround man including man himself. Living things such as bacteria, viruses, insects, rodents and animals produce a variety of diseases and also transmit them. Control of these biological environments can protect the health of an individual and community.

3. Psychological environment: It includes culture, customs, habits, beliefs, attitudes, moral, religion, education, lifestyles etc. These factors also have an impact on health. The principle of health education is to change the attitude and false belief of people about diseases.