Pneumonia is a lung condition wherein the parenchyma of the lung becomes inflamed. The inflammation could occur due to a bacterial, viral, or fungal infection. At times, one may develop this infection after being admitted to a hospital for the treatment of another condition. Under such circumstances, one is diagnosed with hospital-acquired pneumonia. When the infection is acquired outside a hospital, due to contact with an infected individual, one is diagnosed with community-acquired pneumonia. Aspiration pneumonia occurs due to the aspiration of a foreign object or the contents of the stomach into the lower respiratory tract. On the basis of anatomy, pneumonia is classified into lobar, lobular, interstitial, and millary pneumonia. Pneumonia is a common condition that affects about 1 out of 100 people every year. The causative organism in more than half of the cases is a bacterium called Streptococcus pneumoniae. Other bacteria that might be responsible for causing this lung condition include Hemophylus influenza, Moraxella catarrhalis, Mycoplasma pneumoniae, Legionella, Chlamydia, Klebsiella, Staphylococcus aureus, and Pseudomonas aeruginosa. There are times, when a viral infection such as flu might progress into pneumonia. Viruses that could cause this pulmonary infection include Adenovirus, Influenza virus, Respiratory syncytial virus, Hanta virus, Rhinovirus, etc. The virus reaches the lungs through the inhalation of air-borne droplets. After entering the lungs, the virus invades the cell lining of the airways and alveoli. The cells die due to the direct action of the virus or through a cell controlled self-destruction mechanism called apoptosis. As an immune response, white blood cells get activated, which in turn leads to the leakage of fluid into the alveoli. This has an adverse effect on the process of gas exchange in the alveoli. The fungal agents that could cause pneumonia include Histoplasma capsulatum, Cryptococcus neoformans, Pneumocystis jiroveci, blastomyces, and Coccidioides immitis. Toxoplasma gondii, Strongyloides stercoralis, or Ascariasis are some of the parasites that could cause this pulmonary infection. The causative agent or organism gains entry into the body through the respiratory tract by way of inspiration or aspiration of oral secretions. When the organism enters the lungs, the immune system springs into action. The immune system employs the mechanisms of cough reflex, mucociliary transport, or pulmonary macrophages to protect the body against the infection. Problem arise in case of immunocompromised individuals or young children, whose immune system has not yet developed fully. Infection occurs when one's defense mechanism is either suppressed or overwhelmed by the invading agent. The invading organism starts multiplying, thereby releasing toxins that cause inflammation and edema of the lung parenchyma. This leads to the accumulation of cellular debris within the lungs. This leads to consolidation or solidification, which is a term that is used for macroscopic or radiologic appearance of the lungs affected by pneumonia. Bacterial pneumonia is mainly classified into lobar and lobular penumonia. Lobar pneumonia starts in the alveoli and spreads through the pores of Kohn. On the other hand, lobular pneumonia (bronchopneumonia) starts in the terminal and respiratory bronchioles, and spreads through the bronchial walls into the alveoli. In case of lobar pneumonia, there could be homogeneous consolidation of one or more lung lobes. On the other hand, bronchopneumonia is characterized by patchy consolidation of alveolar and bronchial inflammation, often involving both the lower lobes. The stages of lobar pneumonia include: ➠ 24-hour congestion stage ➠ Red hepatization stage ➠ Gray hepatization stage ➠ Resolution stage 24 Hour Congestion Stage This is the first stage that occurs within 24 hours of infection. The lung is affected by vascular congestion and alveolar edema. Microscopic examination shows the presence of many bacteria and a few neutrophils. Red Hepatization The red hepatization stage is observed when the red blood cells and fibrin enter the alveoli. The lung tissue becomes red and firm. This leads to difficulty in breathing or rapid breathing. Gray Hepatization Stage In this stage, fibrin and the dying red and white blood cells collect in the alveolar spaces. The sputum contains a tinge of blood or purulent discharge. Atelectasis, which refers to the reduction of available area within the lung for gas exchange, could also occur. Resolution Stage In this stage, the enzymes in the lungs digest the exudate. The white blood cells fight off the causative organisms and the remains may be coughed up. According to the Centers for Disease Control and Prevention, pneumonia is the leading cause of death in children younger than 5 years of age worldwide. Since flu can progress to pneumonia, precautionary measures should be taken during the time when flu is prevalent. Also, vaccines can be administered to lower the risk. Disclaimer: The information provided in this article is solely for educating the reader. It is not intended to be a substitute for the advice of a medical expert.