Adult post-primary or reactivation tuberculosis
During primary infection, before the development of the immune response, there is lympho-haematogenous dissemination of TB bacilli, most importantly to the apical-posterior areas of the lung*. (There are different theories as to the reason for this apical localisation.) At this site the disease may progress either without interruption or after a latent period of months or years - this results in pulmonary tuberculosis of the adult type (reactivation-type tuberculosis).
Characteristically post primary pulmonary tuberculosis in adults is initially apical and it is usually asymmetrical.
Liquifaction of a caseated area leaves a cavity that does not collapse due to the fibrous capsule.
The bacilli reach extremely high concentrations in a cavity and drainage into the bronchial tree ("endobronchial dissemination") advances the disease into surrounding lung, with further cavitation.
Coughing spreads bacilli to others.
*Except in bats, where it is located at the base of the lower lobe (they spend much of their life hanging upside down).
Chest x-ray - tuberculosis:http://youtu.be/ca1Doezci-w