Concept of the Natural History of Tuberculosis in individuals and community is derived from a large number of studies conducted in India and abroad. The entire course of infection to disease in an individual is divided into five phases which occur at different times subsequent to infection: Phase I of Primary Infection, Phase II of Primary Illness, Phase III of generalised dissemination, Phase IV of localised extra pulmonary tuberculosis and Phase V of Satellite foci or of adult type of disease. The individuals passing through any one or all of the first four phases are incapable of transmission of infection. From the community angle, persons in Phase V with adult type of disease, being the only source of dissemination of infection are responsible to perpetuate the cycle of infection. About 5-8% of the total infected people may develop primary or post primary disease.

Natural History of Tuberculosis in the community also known as epidemiology of tuberculosis aims at understanding the basic laws which govern all the events that take place between tubercle bacilli and the community under natural conditions without active interference in the form of organised control measures. At the start of the principal epidemic wave in a community, the disease takes high toll of children and young adults. A constant feature is the high mortality in males at the two extremes of life, infancy and old age, while in females it is high around 20 years of age. The generalised clinical forms of tuberculosis at the beginning of epidemiological wave and localised chronic disease towards the end of wave are common features. The time span required to attain low levels of prevalence and incidence of infection and disease and mortality are related to the degree of opportunities for transmission of infection and other determinants. The changes in epidemiological situation with relation to time are classified into three phases. i) the epidemic phase (ii) transitional phase and (iii) endemic phase. The epidemic of tuberculosis spans into centuries. The anti-tuberculosis measures specially drugs in particular, have not only changed the outlook for individual patient but by reducing infectivity period, have speeded up the decline of tuberculosis in the community as seen in Japan and Eskimos in Canada. The epidemic course is determined by natural causes which could be modified by human interventions, changes in virulence of agent, susceptibility of host and environmental factors. Tuberculosis is a social disease also and it is essential to create a social environment that wards off infection. Since the tubercle bacilli cannot be extirpated we will have to live with it in symbiosis but keeping it in its place.

The epidemic course of the disease in a particular country can be studied through an epidemic model which is nothing but a mathematical representation of the epidemiological situation in a community. The model is set up by dividing population in various epidemiological classes. The inputs required are: (A) Demographic information, such as (i) division of population into small age groups, (ii) birth rate, (iii) the age-specific death rates. (B) Epidemiological indices such as (i) the division of population by age - the epidemiological classes of: non-infected, infected, inactive lesion, sputum negative active disease and sputum positive active disease, (ii) age and specific incidence of infection and morbidity in various classes, (iii) probability of cure of cases and relapses.

The following information i.e., the tuberculosis situation viz., future prevalence and incidence of the infection, the disease and its trend can be predicted without undertaking repeated surveys. The model could be used for (i) prediction of future tuberculosis situation, (ii) assessment of tuberculosis programme, by matching the actual performance against the predicted natural trend or predicted expectations of the programme, (iii) selection of a suitable anti-tuberculosis programme for problem reduction from amongst a series of alternative programmes, keeping cost in mind, (iv) gathering the type of observation needed for epidemiological studies.