It used to be known as the white death, I suppose to contrast it with the plague, known in Europe as the black death during the terrible epidemics of the Middle Ages. Nonetheless it remains a major problem in many parts of the world, and in some populations even in the U.S. Tuberculosis is typically a disease of the lungs.
Most people catch it by inhaling infectious droplets that someone with the disease has coughed out.
These droplets are tiny and tend to remain suspended in the air if ventilation is not good, waiting to be inhaled. A prolonged exposure to these droplets is usually required for a person to become infected. Being on a subway car where someone is coughing is not a likely scenario for catching the disease. One usually catches it from a family member or friend with whom one spends a fair amount of time. After the germs are inhaled, our body mounts an immune response to the infection which almost always results in the bacteria being walled off in tiny scars in the lungs called granulomas.