The lung can be compared to a tree and its branches with grapes hanging from them . The tree is the wind pipe , the branches bronchus , both with hard walls and the grapes the thin walled alveoli , air filled sacs where carbon dioxide from the body is exchanged for oxygen from the air . If the windpipe becomes inflamed or infected we call it tracheitis and croup sounds , if the branches it is bronchitis and a wheeze can be heard and if the alveoli its alveolitis and one can hear fine crepitations .When secretions fill these alveoli we say the patient has pneumonia . The lung sounds dull on tapping the skin and xrays show white patches of consolidation.
Viruses usually give mild fever , cough and slight chest pain .
Mycoplasma a more persistent dry cough and patchy white infiltrations on the chest x ray, and bacteria usually give greenish or yellowish sputum , fever and general unwellness and solid white areas on the xray. The most common viruses are common cold , and influenza . Sputum examination can sometimes be useful . White count and serology are rarely done. Moraxella is another and in the elderly E Coli due to aspiration .