Pneumonia:

Pneumonia is an inflammation with consolidation  of  the parenchyma of  the lung. It occurs most commonly in infants and young children and may occur as primary disease or as complication  of another illness or may be due to aspiration of  foreign substance. Pneumonia may be classified on anatomic and etiological basis. 

Anatomic:  lobar pneumonia,  lobular pneumonia, interstitial pneumonia, Broncho pneumonia. 

Etiologic: It may be caused by virus; influenze, parainfluenza, adenovirus, mixovirus etc. Bacteria; pneumococci staphylococci, Streptococci, klebseilla, E.coli, pseudomonas etc. Fungi; protozoa, metazoa, chemicals, etc. 

Pathology 

Bacteria reach the lungs through respiratory passages and multiply in the alveoli. The first stage of  attack is called engorgement, during this period the lungs become dark, bluish red and heavy. During the next stage i.e. red hepatization the affected lobes become solid with red cells and fibrin and the air is displaced. In the last stage of grey hepatization  the pleural surface lacks lustre and is dull in colour. The alveoli are  filled with leukocytes  and fibrin. The invading organism produces  inflammation in the mucosa with exudation  in the alveoli  so that it becomes relatively solid.