Tuberculosis does not necessarily occur in healthy people who are infected with the tubercle bacillus. The occurrence of tuberculosis is mainly influenced by two factors, namely, the size of the virulence of the infected tubercle bacillus and the level of resistance of the body. When the human body is first infected with the tuberculosis bacillus, the majority of people usually do not have any symptoms and do not develop tuberculosis. However, in the minority of people infected with the bacillus, TB can occur at any time in their lives when their resistance is reduced. The probability of developing tuberculosis is about 10%.