The natural history of any disease has been divided into two phase’s i.e.
(i) Prepathogenic phase and
(ii) Pathogenic phase
(i) Prepathogenic Phase:
Prepathogenic phase is the period before the onset of disease in man or before the appearance of signs and symptoms of a disease in man. The disease- producing agents of many diseases are present in the surroundings or in the environment but have not yet entered in man.
There are three factors which are necessary for producing the disease which include agent, host and environment. These three factors are known as epidemiological triad.
In the absence of any one of these factors diseases cannot occur. When these three factors are available and the person is susceptible and physically weak then the causative agent will enter into the body to produce a disease.
(ii) Pathogenic Phase:
Pathogenic phase is the period when the disease-producing agent has entered into the body of the host and signs and symptoms of the disease start appearing. In certain cases the causative agent has to multiply to attain strength so as to bring physiological changes or damage the tissues.
The period from the entry of causative agent into the body to the appearance of first symptoms of the disease is known as incubation period.
The incubation period varies from disease to disease. After incubation period it takes some more time that clear cut signs and symptoms of the disease are produced. This period is known as pathogenesis period. During this period if proper treatment is not done it may lead to severe illness, disability of the organ involved or even death of the host.
Every disease has its own natural history and shows characteristic signs and symptoms but it is not necessary that all the individuals will have the same natural history and signs and symptoms of the disease. However by knowing the natural history of the disease one can take firm steps in the prevention, spread and treatment of the disease.