Tuberculosis is an infectious disease caused by tubercle bacillus, Mycobacterium bacillus. In most forms of the disease, the bacillus spreads slowly and widely in the lungs, causing the formation of hard nodules (tubercles) or large, cheese like masses that break down the respiratory tissues and form cavities in the lungs. Blood vessels also can be eroded by the advancing disease, causing the infected person to cough up bright red blood. The tubercle bacillus is a small, rod-shaped bacterium that is extremely hardy; it can survive for months in a state of dryness and can also resist the action of mild disinfectants. Infection spreads primarily by the respiratory route directly from an infected person who discharges live bacilli into the air. Minute droplets ejected by sneezing, coughing, and even talking can contain hundreds of tubercle bacilli that may be inhaled by a healthy person. There the bacilli become trapped in the tissues of the body, are surrounded by immune cells, and finally are sealed up in hard, nodular tubercles. A tubercle usually consists of a center of dead cells and tissues, cheese like in appearance, in which can be found many bacilli. This centre is surrounded by radially arranged phagocytic cells and a periphery containing connective tissue cells. The tubercle thus forms as a result of the body's defensive reaction to the bacilli. Individual tubercles are microscopic in size, but most of the visible manifestations of tuberculosis, from barely visible nodules to large tuberculosis masses, are conglomerations of tubercles. The diagnosis of pulmonary tuberculosis depends on finding tubercle bacilli in the sputum, in the urine, in gastric washings, or in the cerebrospinal fluid. The primary method used to confirm the presence of bacilli is a sputum smear, in which a sputum specimen is smeared onto a slide, stained with a compound that penetrates the organism's cell wall, and examined under a microscope. If bacilli are present, the sputum specimen is cultured on a special medium to determine whether the bacilli are M. tuberculosis. An X-ray of the lungs may show typical shadows caused by tubercular nodules or lesions. The prevention of tuberculosis depends on good hygienic and nutritional conditions and on the identification of infected patients and their early treatment. A vaccine, known as BCG vaccine, is composed of specially weakened tubercle bacilli. Injected into the skin, it causes a local reaction, which confers some immunity to infection by M. tuberculosis for several years. It has been widely used in some countries with success; its use in young children in particular has helped to control infection in the developing world. The main hope of ultimate control, however, lies in preventing exposure to infection, and this means treating infectious patients quickly, possibly in isolation until they are noninfectious. In many developed countries, individuals at risk for tuberculosis, such as health care workers, are regularly given a skin test to show whether they have had a primary infection with the bacillus. Today, the treatment of tuberculosis consists of drug therapy and methods to prevent the spread of infectious bacilli.