The primary means of transmission of tuberculosis is airborne. The main way of spreading tuberculosis is through airborne transmission. When a person with tuberculosis coughs and sneezes, the tubercle bacilli are suspended in the air and spread by droplets, which can cause infection when inhaled by healthy people. When sputum coughed up by a TB patient is exposed to the air, Mycobacterium tuberculosis will be mixed into the dust in the air and can also be infected by inhalation. Other routes of infection are also possible, such as gastrointestinal infection from drinking infected milk, mother-to-child transmission from a pregnant woman with the disease, and infection from skin wounds, but these are less common. Sputum from patients with open TB is the main source of infection. With regular chemotherapy, the infectivity decreases as the amount of sputum expelled decreases.