This paper provides a concise overview of pneumonia, a lung infection caused by bacteria, viruses, or fungi that remains the leading infectious cause of hospitalization and death in the United States. It examines the primary sources of infection — including self-infection via compromised oral hygiene and immune function — and identifies the most common pathogens in both community and hospital settings. The paper then explains the pathophysiology of pneumonia, detailing how bacterial, viral, and fungal infections affect the alveoli and impair gas exchange. Finally, it addresses prevention strategies and the relative severity of each pneumonia type, noting the particular dangers of viral pneumonia and the mildness of mycoplasma ("walking pneumonia") infections.
Pneumonia is a lung infection caused by bacteria, viruses, or fungi. The most direct endogenous sources of these microorganisms include infected nasal carriers, sinusitis, oropharyngeal colonization, gastric or tracheal colonization, and hematogenous spread (Alcón Fàbregas & Torres, 2005). Although much is known about pneumonia, it remains the leading infectious cause of hospitalization and death in the United States ("Pneumonia," n.d.). One reason pneumonia is so common is that the lungs are exposed to roughly ten thousand liters of air each day, as well as small amounts of saliva and occasionally aspirated regurgitation — all of which can carry bacteria, viral and fungal agents, and other pathogens ("Pneumonia," n.d.).
Although pneumonia is contagious via airborne particles from infected persons, self-infection is far more common. This is why oral hygiene, nutrition, and other lifestyle factors are central to reducing disease prevalence. When the immune system has already been compromised, the risk for pneumonia rises considerably. The most common bacteria causing pneumonia outside of the hospital setting include Streptococcus pneumoniae (pneumococcus) and Haemophilus influenzae. In hospital settings, the most common pneumonia-causing bacteria include Staphylococcus aureus, Klebsiella pneumoniae, Pseudomonas aeruginosa, and Escherichia coli — all of which are highly resilient ("Pneumonia," n.d.).
It is usually not possible to determine which specific pathogen is responsible for a given infection, which is why broad-spectrum antibacterial agents can address most bacterial cases. In healthy individuals, these microbes generally do not reach the alveoli in the lungs, thanks to protective processes such as coughing; and if they do reach the alveoli, immune cells can typically clear the infection. Unfortunately, viral infections causing pneumonia are more difficult to treat ("Pneumonia," n.d.), and more effective antiviral medications are needed to minimize infections and prevent fatalities.
In hospitalized patients, the immune system is frequently compromised, leading to elevated infection rates. At-risk populations are particularly vulnerable upon contact with viral forms of pneumonia. Prevention strategies in the general population include maintaining oral hygiene, good nutrition, regular aerobic exercise, and refraining from smoking ("Pneumonia," n.d.). Once an infection has been detected, antibiotics are indicated for bacterial cases. The lungs react to a bacterial infection in various ways, the most notable of which is capillary leakage — allowing protein-rich fluids to enter the alveoli ("Pneumonia — Pathophysiology of Pneumonia," n.d.).
When the normally air-filled alveoli are flooded with fluid, the lungs cannot carry out their oxygen–carbon dioxide exchange efficiently, causing oxygen deprivation and/or carbon dioxide poisoning. Simultaneously, the lungs produce excess mucus to ward off infection, and the leaky capillaries may seep blood into that mucus. White blood cell accumulation further impedes alveolar function. When the alveoli become sufficiently filled with excess fluid, the respiratory system can shut down entirely.
"Alveolar fluid buildup and impaired gas exchange"
"Viral, mycoplasma, and fungal pneumonia compared"
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