Transmission

TB is mostly an airborne disease. The disease is not likely to be transmitted through personal items belonging to those with TB, such as clothing, bedding, or other items they have touched. Adequate ventilation is the most important measure to prevent the transmission of TB. Because most infected people expel relatively few bacilli, transmission of TB usually occurs only after prolonged exposure to someone with active TB. On average, people have a 50 percent chance of becoming infected with TB if they spend eight hours a day for six months or 24 hours a day for two months working or living with someone with active TB, researchers have estimated.

People are most likely to be contagious when their sputum (mucoid matter ejected from the lungs, bronchi, and trachea, through the mouth, not to be confused with saliva, as sputum is the mucoid matter originating from deeper within the respiratory system) contains bacilli, when they cough frequently and when the extent of their lung disease, as revealed by a chest x-ray, is great. TB is spread from person to person in microscopic droplets - droplet nuclei - expelled from the lungs when a TB sufferer coughs, sneezes, speaks, sings, or laughs. Only people with active disease are contagious.

Droplet nuclei are tiny and may remain in the air for prolonged periods, ready to be inhaled. They are small enough to bypass the natural defences of upper respiratory passages, such as hairs in the nose or the hair like cilia in the bronchial tubes. Infection begins when the bacilli reach the tiny air sacs of the lungs known as alveoli, where they multiply within macrophages (white blood cells).
People who have been treated with appropriate drugs for at least two weeks usually are not infectious.