All who escape untimely death from other causes are infected with tubercle bacilli. The majority are insusceptible and develop no recognized evidence of infection. The minority are of three grades of susceptibility. Those of least susceptibility recover spontaneously, often despite maltreatment. The moderately susceptible may recover if aided promptly and properly, or may arrest the progress of the disease spontaneously. The disease in the most susceptible may be arrested if treatment is favorable, or its progress may be only retarded.
The strength or weakness of resistance, defense and repair determines insusceptibility and the grades of susceptibility. Because they are the products of activities of cells, they are not static but dynamic, and are constantly fluctuating. Fluctuations are irregularly periodic and tend to take two courses. The more common course is downward. Through fatigue and exhaustion caused by unhygienic living, by intercurrent diseases and by age, people tend to forfeit more