Pneumonia Overview and Causes

An infection of the lower respiratory tract caused by various pathogens, including bacteria, viruses, fungi, protozoa, and parasites.

Pneumonia acquired in the community, outside of a healthcare setting.

The most common cause of Community-Acquired Pneumonia (CAP).

Pneumonia developed in a healthcare setting, usually 48 hours or more after admission.

Pneumonia specifically occurring in patients who are intubated and mechanically ventilated.

The initial step in the development of pneumonia, where the pathogen enters the lower respiratory tract.

The immune system's recognition of the pathogen in the lungs, involving alveolar macrophages and their response.

A type of immune cell found in the alveoli that plays a key role in recognizing and fighting pathogens.

The longer a patient stays in the hospital, especially in an ICU, the higher their risk of developing HAP.

Patients on ventilators have a significantly higher chance of developing VAP, which is a type of HAP.

Since bedridden patients have reduced lung expansion, they are more prone to bacterial growth and infection.

Invasive medical devices like catheters and tubes increase the risk of infection by providing entry points for bacteria.

Previous antibiotic use alters the natural balance of bacteria, making the patient susceptible to antibiotic-resistant organisms like MRSA.

Lying flat on the back restricts lung expansion, making it easier for bacteria to accumulate in the lungs.

Poor oral hygiene allows bacteria to accumulate in the mouth, increasing the risk of inhaling them into the lungs.

People with weakened immune systems (HIV/AIDS, cancer treatment, organ transplant) are more susceptible to opportunistic infections like pneumonia.

Neutrophils, a type of white blood cell, arrive at infected alveoli in the lungs and release substances to fight the infection. These include antimicrobial proteins to kill microbes, toxic free radicals to damage pathogens, neutrophil extracellular traps (NETs) to capture microbes, and inflammatory mediators to signal other immune cells.

The release of toxic substances by neutrophils, such as free radicals, can damage the delicate lining of the airways and blood vessels in the lungs, leading to fluid leakage into the alveoli.

Fluid and debris accumulating in the alveoli reduces ventilation, leading to a mismatch between the amount of air reaching the alveoli and the blood flow passing through them (ventilation-perfusion, V/Q mismatch).

Low blood oxygen levels (hypoxemia) and difficulty breathing (dyspnea) are the consequences of impaired gas exchange in pneumonia.

Bacterial pneumonia spreads through respiratory droplets released when an infected person coughs or sneezes, or by contact with contaminated surfaces.

Viral pneumonia spreads through aerosolized droplets released from infected people when they sneeze, cough, or even talk.

Hospital-acquired pneumonia (HAP) and ventilator-associated pneumonia (VAP) are caused by microbes introduced through contaminated medical equipment or cross-contamination in healthcare settings. These often occur in patients already weakened by other illnesses.

People with weakened immune systems are more vulnerable to pneumonia. This includes individuals with HIV, cancer, organ transplants, or those taking corticosteroids. The type of organism causing pneumonia and its mode of transmission vary in these cases.