How can someone contract tuberculosis and what does it do to that person? why is it considered a 'lung disease'?

Inhaled infection: Tb generally enters the body by inhalation of droplets that were coughed out by an infected person around you. Once you inhale the droplets containing TB bug, it starts to multiply in your lungs & causes infection. It can also spread to lymph nodes & via bloodstream to other organs in the body but majority of infections are limited to lungs (hence considered a lung disease).