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TB is caused by a bacterial infection. The bacteria is carried/transmitted by airborne droplets. Once inside the body macrophages engulf the virus and attempt to destroy it. If this fails the bacteria can create a local lung infection. The immune response is to isolate the bacteria by forming connective scar tissue around it forcing it into an inactive state. This calcified fibrosis reduces the surface area for gas exchange and form tubercles, hence the name.
If the body's immune system is weakened by old age, HIV, cancer or drug abuse, TB can spread to other organs such as the lining of the brain, the kidneys and spinal chord.
Short term symptons include, fever, coughing, weight loss, loss of apetite.
Long term, coughing up blood smeared sputum, tightness in chest, breathlessness.