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The risk of contracting TB upon exposure depends on the concentration of the infectious particles surrounding a susceptible individual, the duration of exposure time, and the closeness of contact with infectious persons, especially in closed or poorly ventilated environments. Those individuals who become infected with M. tuberculosis develop a specific cell-mediated immunity responsible for effective control of infection and a positive reaction to the tuberculin skin test between two and ten weeks after infection. Individuals with tuberculous infection but not active disease are not infectious and usually are asymptomatic, but approximately 10 percent remain capable of developing disease within their lifetime, and 5 percent within the first two years of infection.