TB is not a disease, which will affect only the higher upper class or the lower class. It can really infect any person, devoid of any class. The most important determinants of contagion are closeness of contact and infectiousness of the source, which vary widely among individuals. In order to be infectious, a patient must have a sufficient concentration of microorganisms in the sputum to create the floating, infecting droplet nuclei while coughing or sneezing. The most important fact is that the patient should have pulmonary TB. Given a contact with an infectious person, on an average, 1 in 6 persons will become infected.

BCG vaccination also plays an important role here. It is seen that 54% of unvaccinated individual adolescents developed clinical TB within a year and 78% within 2 years. The risk of progression of infection to active disease is higher in young children, over 50% of those infected being under the age of 6 months. Children between 3-4 years of age and puberty, although as susceptible to infection as younger children, develop the disease infrequently. In adolescents and early adulthood, more often in females than in males, the risk of recent infection producing active disease is much greater. Moreover, infections which are acquired and remain latent during childhood but become active in adolescence or early adulthood, are due to altered immunologic factors like delayed type hypersensitivity (DTH) with increasing tendency to tissue necrosis.

Middle aged adults are less susceptible to progression of infection to disease. However in old age, a decreased resistance develops more in men than women. The likelihood of active disease varies with intensity and duration of exposure. Other factors like tall, thin people and black men with histocompatibility type HLA-Bw15 are said to be at increased risk independent of more important social factors.

Malnutrition, intercurrent situations like alcoholism, homelessness, imprisonment, immunosuppression and AIDS greatly favour progression of infection to active disease.