Q: I have been diagnosed with Koch’s infection (TB of the bone) & am on anti TB drugs. How does one get infected with TB of the bone? Is it caused due to inhalation of a droplet of the cough /sneeze from a TB patient? I lead a very healthy lifestyle with respect to food, sleep & exercise. I have heard that TB is generally caused due to poor sanitation, hygiene, etc. How can I be infected in spite if leading a healthy lifestyle?

A:TB occurs when individuals inhale the bacteria aerosolised by infected patients. The infective droplet nucleus is very small, measuring 5 micrometres or less, and may contain approximately 1-10 bacilli. Although a single organism may cause disease, 5-200 inhaled bacilli are usually necessary for infection. The small size of the droplets allows them to remain suspended in the air for a prolonged period of time. The number of bacilli in the aerosol and the relative virulence of the organism are the major factors determining transmission of the disease. The infectiousness of the source case is very important in transmission. Also, contacts of persons with sputum-positive smears have an increased prevalence of infection as opposed to contacts of those with sputum-negative smears. 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, congregation in prison settings, poor housing, and inadequate ventilation predispose individuals to the development of TB. The bacilli do not always respect the social hierarchy and even the rich do fall prey to this infection, despite leading a healthy lifestyle. Primary infection of the respiratory tract occurs as a result of inhalation of these aerosols. Upon inhalation, the bacilli are deposited in the lungs. Subsequently, the macrophages ingest the bacilli but are unable to kill them, and the bacilli continue to multiply. TB bacilli are slow growing and tolerate the intracellular environment, where they may remain metabolically inert for years before reactivation and disease. Bacilli may remain dormant in the apical posterior areas of the lung for several months or years, with later progression of disease resulting in the development of reactivation-type TB (i.e. endogenous re-infection TB). Transportation of the infected macrophages to the regional lymph nodes can occur, from where lymphatic spread occurs to other lymph nodes, kidney, bones, vertebral bodies, and meninges leading to peripheral lymphadenopathy, tubercular meningitis, miliary TB, bone TB, and other organ involvement. The common sites involved in bone or joint TB is the large weight bearing bones or joints including the vertebrae (50%), hip (15%), and knee (15%). Destruction of the bones with deformity is a late sign of TB. Most persons infected with M. tuberculosis do not develop active disease. In healthy individuals, the lifetime risk of developing disease is 5-10%. TB disease may develop in certain instances, such as extremes of age or defects in cell-mediated immunity (e.g. HIV/AIDS infection, malnutrition, chemotherapy for cancers, prolonged steroid use). For patients with HIV infection, the risk of developing TB is 7-10% per year. You have not mentioned how TB was diagnosed in your case, but if your doctor has diagnosed it on the basis of clinical grounds supported with lab evidence, and then please ensure that you take the medicines for the duration advised for a complete cure.