I am often told by patients that I have no TB bacteria in my sputum and am not infectious. This is not true. This part of the patient’s sputum may still have live TB bacteria in it. A positive sputum tuberculosis smear is one of the gold standards for the diagnosis of tuberculosis. About 30 out of 100 TB patients are sputum positive for TB bacteria. This means that about 70% of TB patients are sputum negative. Only this proportion of patients are not able to detect TB bacilli with the current examination technology. The amount of tuberculosis bacteria is too small. The test technique is not sensitive enough. There are other factors that may contribute to this result, such as the fact that the sputum specimen taken from the patient is oral sputum, not sputum from the lungs. A positive sputum TB culture is another gold standard for the diagnosis of TB. Tuberculosis bacilli may be cultured in about 40 out of 100 patients with tuberculosis. About 60% of TB patients have a negative sputum TB culture. Because not every patient has sputum culture for TB, sputum culture for TB is expensive and takes a long time. It used to take more than 1 month, and now it takes about 10 days. Also, not every hospital can do sputum culture for TB, and most hospitals do not perform this test. Even if the sputum culture is negative, it is still not possible to say that it is not infectious because the number of TB bacteria needs to be 1×102/ml to have a positive result. In addition, the TB bacilli in the sputum taken may have been disposed of improperly and may have died, so the culture does not come out. Therefore, it is still possible to have a negative result due to technical and other reasons. There was a patient who needed surgery for kidney disease and was diagnosed with “old tuberculosis” on the chest X-ray before surgery, but the lung was originally thought to be fine and surgery was planned. Later, he was referred to our hospital for other reasons, and his sputum culture was positive for tuberculosis. Therefore, the diagnosis of “old tuberculosis” should not be made easily for untreated tuberculosis. The presence or absence of TB bacilli in sputum is based on the number of TB bacilli that can be detected in a certain amount of sputum sample exceeding a certain number of sets. If the number of TB bacteria in the sputum is lower than this number, the sputum will be negative for TB bacteria, and there are other factors that may cause this negative result. It is easy to think that there is no infectiousness, but in fact these patients may still be emitting TB bacteria from their sputum. Therefore, appropriate protective measures are still needed at this time, such as wearing a mask, disinfecting the air or patient supplies with ultraviolet light, taking care to cough protectively when the patient coughs, and not speaking loudly or spluttering when speaking. It is necessary to take a chest X-ray of the patient’s close living person because the close living person may be infected before the patient is diagnosed with tuberculosis. Tuberculosis bacilli are transmitted through the air, and as long as the air contains droplets of tuberculosis bacilli, once inhaled by a healthy person, he may develop tuberculosis. Transmission of tuberculosis bacilli