TB carriers are the infectious agents of TB transmission, and it is mainly through positive sputum tests that carriers of TB disease can be detected. In contrast, tuberculosis outside the lungs, such as lymph node tuberculosis and tuberculous pleurisy, is not contagious. TB bacilli can be spread by coughing and sneezing into the air by TB patients, and can thus be spread by droplets and airborne transmission, and anyone who inhales these droplets will be called a contact. If the contact person has normal immunity, he or she does not develop active TB. For people with low immunity who are infected with TB, active TB infection can occur.