In patients with pulmonary tuberculosis, sputum from the respiratory tract is negative for Mycobacterium tuberculosis and cannot be found, but if the sputum is taken again and the culture is positive for Mycobacterium tuberculosis, it is still infectious. This is mainly because the number of Mycobacterium tuberculosis in the sputum is relatively small and the concentration is relatively low, and it is impossible to find the antacid bacilli from the sputum alone. In this case, the most reliable evidence of infectivity is the presence of Mycobacterium tuberculosis through culture. In this case, the patient can transmit the infection to the surrounding healthy people through the respiratory tract by droplet route, so isolation treatment is still needed.