Tuberculosis is transmitted mainly by droplet infection and droplet nuclei generated when a patient who has tuberculosis coughs. Coughing generates the largest number of droplets of all sizes. To transmit infection, the particles must be fresh enough to carry a viable organism. The frequency and vigor of cough and the ventilation of the environment influence transmission of the infection. Tuberculosis is not transmitted by fomites, such as dishes and other articles used by the patients.

Incubation period of Tuberculosis
The time from the receipt of infection to the development of a positive tuberculosis test ranges from three to six weeks, and thereafter, the development of the disease depends upon the closeness of contact, extent of the disease and sputum positivity of the source case (dose of infection) and host–parasite relationship. Thus, the incubation period may be weeks, months or years.

FAQs on Tuberculosis
Some common questions
Does tuberculosis affect any particular age group?
No. Tuberculosis affects all ages. The infection can be seen in infants, children as well as adults.
Is it true that tuberculosis affects men more than women?
Yes, tuberculosis affects men more than women.
Is tuberculosis heredity?
Tuberculosis was once thought to be a hereditary disease. This, however, is not true.

Immunity against tuberculosis
Man has no inherited immunity against tuberculosis. As mentioned earlier, immunity develops when one is exposed to the disease or gets the BCG vaccination. It is still possible to get the disease despite the development of this immunity, if the immune system gets compromised i.e. patients with cancer, those on chemotherapy or steroids, etc.
Skin tests for diagnosing tuberculosis
Skin testing for tuberculosis was discovered by Von Pirquet in 1907. The substance that is injected into the skin is a purified protein derivative (PPD) of the organism. The most widely used is the Mantoux test.

Mantoux test for Tuberculosis
The Mantoux test is carried out by injecting intradermally (within the dermis layer of the skin) on the forearm. The strength of the amount injected is expressed as tuberculin unit (TU). For the Mantoux test, 1TU strength is used. A control substance is used on the opposite forearm. It usually consists of a fungal derivative. It serves as a control.

Interpretation of the test
The test is interpreted 48 hours after the substance has been injected, as this is the time required for the body to mount an immune response. Tuberculin reaction consists of redness and hardening of the skin. Skins that are red are sometimes difficult to measure. The area of hardening alone is measured by using a plastic ruler or calipers. The horizontal transverse diameter is measured.
Reactions exceeding 10 mm: considered “Positive”.
< 6mm: Test is negative.
6–9 mm: The test is indeterminate. It is possible that infection is due to tuberculosis or a related organism. The reaction may be due to the M.tuberculosis or a typical mycobacteria.

Reliability of the skin test
Not necessarily. The test is positive in people exposed as well as people who have the infection. In Western countries, since exposure to tuberculosis is not so common, it is more likely that the patient has an infection. If the test is strongly positive i.e. the hardened area is more than 15mm and the person has had symptoms suggestive of tuberculosis, then it is more likely that the person has tuberculosis.