Tuberculosis can be transmitted early. The susceptible groups of TB are HIV-infected patients, diabetics, pneumoconiosis patients, immunosuppressant users, and the elderly who are immunocompromised. The main way of transmission of TB is through the respiratory tract, via respiratory droplets. When a TB patient coughs, sneezes, laughs, talks and sings, microdroplets containing Mycobacterium tuberculosis can be spread from the respiratory tract into the air and can stay there for several hours, causing infection if inhaled by others. Other routes include drinking milk with the bacteria, infection through the digestive tract, mother-to-child transmission through the placenta in pregnant women, infection through skin wounds and direct contact with the upper respiratory tract. Of course, not all TB patients are contagious, and some patients who are sputum negative or who have been effectively treated for half a month may not be contagious.