There is no correspondence between the length of contact with a patient with tuberculosis and whether he or she is infectious. If the patient is stable and not obviously infectious, he or she will not be infected no matter how long he or she is exposed. If the patient is seriously ill and has cavities in his or her lungs, he or she will be highly contagious and may be infected if he or she passes in front of the patient. Since tuberculosis is a contagious disease, minimizing contact with patients with tuberculosis in the contagious stage can greatly reduce the risk of being infected. The fact that Mycobacterium tuberculosis can be detected in the sputum of TB patients indicates that they are infectious. Multiple cavities of varying sizes form in the lungs of TB patients, and the patients are extremely contagious. When it is necessary to have contact with a TB patient in the infectious stage, it is important to protect yourself from being infected, and TB patients should exercise self-discipline and refrain from going to public places.