The prevalence and spread of infectious diseases require three factors: the source of infection, the route of transmission, and the susceptible population. So preventing the transmission of Mycobacterium tuberculosis also requires these three aspects. Some patients are more infectious, such as those with cavitary active TB, while others are less infectious or non-infectious, such as those who are sputum-negative. TB infections in other parts of the body, such as TB pleurisy, kidney TB, and TB meningitis, are not contagious to those around them. First of all, the source of infection. The main source of infection for tuberculosis is the patient with secondary tuberculosis. Since Mycobacterium tuberculosis is mainly disseminated with sputum, patients who have Mycobacterium tuberculosis in their sputum are infectious and are the source of infection. The degree of contagiousness depends on the amount of bacilli in the sputum. How can you tell if a patient with tuberculosis is contagious? If Mycobacterium tuberculosis is detected by the direct smear method, it is a large number of bacteria, and if the direct smear method is negative and only Mycobacterium tuberculosis is cultured, it is a small amount of bacteria. Next, we will talk about the transmission route. Mycobacterium tuberculosis is mainly spread by coughing, sneezing, laughing, and talking loudly, and by expelling droplets containing Mycobacterium tuberculosis into the air. Droplet transmission is the most important route of transmission of tuberculosis. Transmission by other routes, such as the gastrointestinal tract and skin, is now rare. Again, a word about susceptible populations. Factors affecting the body’s natural resistance to Mycobacterium tuberculosis include, in addition to genetic factors, social factors such as living in poverty, crowded housing, and poor nutrition. Infants and children with an imperfect cellular immune system, the elderly, HIV-infected patients, immunosuppressant users, and patients with chronic diseases with low immunity are all susceptible populations to TB. Finally, a brief note on the factors affecting infectivity. The size of infectiousness depends on the amount of Mycobacterium tuberculosis excreted by the patient, the density of space containing Mycobacterium tuberculosis microdroplets and ventilation, the closeness and duration of contact, and the status of individual immunity. Ventilation to reduce the density of spatial microdroplets is an effective measure to reduce the transmission of tuberculosis. Of course, the most fundamental way to reduce the number of spatial microdroplets is to cure TB patients.