It is true that one-twelfth of the total deaths from all causes are due to phthisis, and that at certain ages as many as half the bodies of persons having died from other diseases have been found to present old healed or latent tuberculous lesions. Consumptives differ greatly in infectivity. Only a few bacilli reach the experimentally determined duration of extra-corporeal existence. Direct sunlight kills them quickly, being a disinfectant without peer. The dissemination of the infectious material discharged by consumptives is limited by its physical character. If the patient and his attendants and friends take the simple precautions required to prevent spray infection during the act of coughing, no immediate danger attaches to the expectoration. Even when expectoration becomes dust, and the particulate infective material can be scattered, it obeys the laws of gravity and tends to sink again after being disturbed. The air expired by consumptives in ordinary breathing is absolutely sterile.