Tuberculosis. Coloured X-ray of a human chest with pulmonary tuberculosis in the patient's left lung (fluffy yellow areas). The disease is caused by the bacterium Mycobacterium tuberculosis. When inhaled into the lungs, the bacteria form a primary tubercle (nodular lesion). They then spread to the lymph nodes. This form is called pulmonary tuberculosis or consumption & phthisis (wasting). Many people are infected without showing symptoms. Despite the immune system's ability to hold the disease in check, these people remain carriers, transmitting the bacilli by coughing or sneezing. Symptoms include fever, weight loss, night sweats & spitting of blood.