False colour human chest x-ray showing evidence of tissue calcification from pulmonary tuberculosis (TB) in both lungs (yellow patches). Pulmonary TB is caused by the bacterium Mycobacterium tuberculosis, which when inhaled into the lungs forms a primary tubercle (nodular lesion), & spreads via the nearest lymph nodes. 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 & the spitting of blood. Secondary spread of disease may give rise to tuberculous meningitis.