Pneumonia has been defined in several ways throughout the world. In less-developed parts of the world, evidence of retractions along with tachypnea leads to a clinical diagnosis of pneumonia. In areas that have more technology available, the presence of infiltrates on a chest radiograph is needed to diagnose pneumonia formally.[1] Regardless of how diagnosed, however, pneumonia in infants and children is a common cause for families to seek medical attention. In addition, infections of the lower respiratory tract have varying morbidity and mortality when comparing illness in developed and developing countries. The combined effects of malnutrition and inadequate immunization may cause much more severe respiratory disease for children who also may not have the benefits of advanced therapy to treat the underlying infection and subsequent complications. This circumstance, then, leads to many more deaths caused by pneumonia in the developing world. However, the general concepts regarding pneumonia are the same and are reviewed here.

The incidence of acute pneumonia varies by age. Infants and toddlers are more commonly infected by respiratory pathogens than older children. Children in the first 5 years of life have an incidence of 30 to 45 episodes of acute lower respiratory illness per 1000 children per year. This figure drops to approximately 16 to 20 cases in the 5- to 9-year-old age group. In older children and adolescents the incidence of pneumonia is estimated to be 6 to 12 cases per 1000 patients.[2] Many risk factors have been investigated that may alter the risk for developing pneumonia. Importantly, exposure to environmental tobacco smoke and air pollutants has been shown to be associated with more than 190,000 cases of pneumonia per year in the United States among persons in the youngest age groups. Other factors that may increase the risk of pneumonia include malnutrition, immune deficiency, or severe developmental delay. Infants who have a history of prematurity and chronic lung disease may also be at risk for acute lower respiratory infection.

Infection of the lower airways occurs as a result of introduction of an overwhelming load of pathogenic organisms or a breach of host defense mechanisms. Certainly well known is the fact that particular organisms can invade the lower airways, causing a significant lower respiratory tract infection. Mechanisms of host defense that provide protection to the lung include the ciliary elevator, which serves to trap foreign material that is inhaled and then removed on a continuously produced thin layer of mucus. Ciliary dysfunction or loss is part of viral infection and may allow colonizing bacteria to gain entry to the lung parenchyma, allowing pneumonia to develop. Loss of the normal cellular components of the immune system (polymorphonuclear leukocytes and alveolar macrophages) caused by either acquired or congenital immune deficits are well known to place patients at risk for lung infection. Immunoglobulin is also vital in protection against pneumonia. Aspiration of foreign material, either from the oral cavity in the form of excess saliva or food contents, may lead to pneumonia. Pathologically, increased numbers of inflammatory cells, organisms, and alveolar fluid are seen during the course of illness. Early on, high numbers of pathogens are seen; during the resolution phase, inflammatory cells abound as the infectious process is contained and cleared.[3]