OVERVIEW OF PNEUMONIA
Infections of the lower respiratory tract may involve the airways, lung parenchyma, or pleural space. Pneumonia is an infection of the gas exchanging units of the lung, most commonly caused by bacteria, but occasionally by viruses, fungi, parasites, or other infectious agents. In immunocompetent individuals, pneumonia is characterized by a brisk ﬁlling of the alveolar space with inﬂammatory cells and ﬂuid. If the alveolar infection involves an entire anatomic lobe of the lung, it is termed lobar pneumonia, and some episodes may lead to multilobar illness and more severe clinical manifestations. When the alveolar process occurs in a distribution that is patchy and adjacent to bronchi, without ﬁlling an entire lobe, it is termed bronchopneumonia.
Based on clinical presentation, pneumonias have also been classiﬁed as being typical or atypical. The typical pneumonia syndrome is characterized by a sudden onset of high fever, shaking chills, pleuritic chest pain, and productive cough, and it can be expected only if the patient has an intact immune response system and if the infection is caused by a bacterial pathogen such as Streptococcus pneumoniae, Haemophilus inﬂuenzae, Klebsiella pneumoniae, Staphylococcus aureus, aerobic gram-negative bacilli, or anaerobes. If a patient is infected by one of these organisms but has an impaired immune response, the classic pneumonia symptoms may be absent, as can be the case in elderly and debilitated patients. The atypical pneumonia syndrome, characterized by preceding upper respiratory symptoms, fever without chills, nonproductive cough, headache, myalgias, and mild leukocytosis, is often the result of infection with viruses, Mycoplasma pneumoniae, Chlamydophila pneumoniae, Legionella organisms, and other unusual infectious agents (as in psittacosis and Q fever). In clinical practice, it is often very difﬁcult to use clinical features to predict the microbial cause of pneumonia.
When a parenchymal lung infection leads to break-down of lung tissue, it may cause tissue necrosis and cavity formation, and this type of infection is termed a lung abscess. These infections usually result when a patient aspirates a highly virulent pathogen into the lung in the absence of effective clearance mechanisms; the etiologic agents include S. aureus, K. pneumoniae, Escherichia coli, and Pseudomonas aeruginosa. Empyema is an infection of the pleural space characterized by grossly purulent material that is usually caused by extension of parenchymal infection outside the lung; it is caused by anaerobes, gram-negative bacilli, S. aureus, and occasionally tuberculosis (TB).
Another classiﬁcation system that is applied to pneumonia relates to the place of origin of the infection. When the infection occurs in patients who are living in the community, it is termed community-acquired pneumonia (CAP), although it is called nosocomial pneumonia, or hospital–acquired pneumonia (HAP) if it arises in a patient who is already in the hospital. When HAP develops in a patient who has been on mechanical ventilation for at least 48 hours, it is termed ventilator-associated pneumonia (VAP). The distinction between CAP and HAP is becoming increasingly blurred because of the complexity of patients who reside out of the hospital. When pneumonia develops in patients who come from a nursing home, in those receiving chronic hemodialysis, and in those admitted to the hospital in the past 3 months, it is termed health care–associated pneumonia (HCAP). Because of their contact with the health care environment, these patients may already be colonized with multidrug-resistant organisms when they arrive at the hospital. Thus, the relationship between bacteriology and the place of origin of infection is a reﬂection of several factors, including the comorbid illnesses present in the patient, their host-defense status, and their environmental exposure to speciﬁc pathogens.
Patients who develop pneumonia while receiving immunosuppressive therapy or who have an abnormal immune system are referred to as compromised hosts, and the infectious possibilities vary with the localization of the immune defect. In recent years, particularly with the application of immunosuppressive therapy for a variety of illnesses, with the emergence of AIDS, and with an increasing number of institutionalized elderly individuals, TB, fungal, and parasitic lung infections have reemerged as important and common infections.