Aetiologically Pneumonia can be divided into two groups :
- The Specific Pneumonia : In which the disease is caused by a definite or specific pathogenic organism.
- The Aspiration Pneumonia : In these some abnormality in the respiratory system predisposes to the invasion of the lung by organisms of relatively low virulence such as - H.Influenzae, some Streptococcal Pneumonae and certain of the bacteria forming the normal flora of the upper respiratory tract and mouth. In this group of pneumonia as term implies, infection generally reaches the alveoli by aspiration from other parts of the respiratory tract.
By and large pneumonia is defined as inflammation in the lung parenchyma, the portion distal to the terminal bronchioles and comprising the respiratory bronchioles, alveolar ducts, alveolar sacs and alveoli. While the inflammation may have many different causes and varying durations, the term pneumonia commonly refers to acute infection. The causative organisms reach the lung by any of these roots which are :
- Inhalation of microbes present in the air.
- Aspiration of organisms from the naso-oropharynx, interestingly is the most common cause of bacterial pneumonia.
- Haematogenous spread from a distant focus of infection,or rarely
- Direct spread from a contagious site of infection.
The group of Specific pneumonia may further be subdivided into Bacterial and Viral Pneumonias. Generally Streptococcus Pneumonae, Staphylococcus Pyogens, Klebsiella Pneumonia and Mycobacterium Tuberculae are the bacteria known to cause bacterial pneumonias
