Q: My one-year-old son is active and healthy. Recently a Mantoux test was done and the result showed induration of 6 x 6mm. The family doctor says its OK. My brother in law caught TB and says its fine and cured after a years treatment. He has stopped taking medicines. He often comes to our house and plays with my son. I have already warned him and told him to be away from the child. What should I do under these circumstances? What are the chances of infection?

A:TB is transmitted by inhaling the tubercle bacilli. The number of bacilli in the inoculum and the relative virulence of the organism are the major factors determining transmission of the disease. The infectiousness of the source case is of vital importance in determining likelihood of transmission. Bacillary population of TB lesions varies and depends on the morphology of the lesion. Nodular lesions have 100-10,000 organisms, whereas cavitary lesions have 10 million to 1 billion bacilli. Thus, persons with cavitary lesions have the potential for being highly infectious. Also, contacts of persons with sputum-positive smears have an increased prevalence of infection as opposed to contacts of those with sputum-negative smears. Persons who have received anti-TB drugs are much less infectious than those who have not received any treatment. This decline in infectiousness is due primarily to reduction in the bacillary population in the lungs. Environmental factors also contribute to the likelihood of acquiring the infection. The concentration of bacilli depends on the ventilation of the surroundings and exposure to ultraviolet light. Thus, overcrowding, poor housing, and inadequate ventilation predispose individuals to the disease. As you can appreciate, there is no risk of your child getting the disease from his uncle who has been treated for a year and is not transmitting the bacteria in his sputum.