From airway to air sac
Pneumonia is infection of the lung tissue itself — specifically the alveoli, the tiny air sacs where gas exchange happens. In bronchitis the airways are irritated but air still reaches the alveoli; in pneumonia the air sacs fill with fluid, pus, and immune cells. On a chest X-ray this filled-in lung shows up as a white patch called consolidation. That patch is the difference, and it is why pneumonia can genuinely starve the blood of oxygen while bronchitis usually does not.
When fluid fills alveoli, doctors listening with a stethoscope hear crackles — a fine popping like Velcro pulling apart — over the affected area. Because air no longer flows freely there, oxygen cannot cross into the blood, producing hypoxemia (low blood oxygen) that a pulse oximeter on the finger can detect.
Community-acquired pneumonia and its usual suspects
Community-acquired pneumonia (CAP) is pneumonia caught in everyday life, outside hospital. The single most common bacterial cause is pneumococcus (Streptococcus pneumoniae). It classically produces a sudden high fever, shaking chills, a cough with rusty-coloured sputum, and sharp pleuritic chest pain that stabs when you breathe in. Viruses such as influenza and COVID-19 also cause pneumonia, sometimes followed by a bacterial infection on top.
Diagnosis usually rests on the combination of symptoms, crackles or dullness on examination, and consolidation on the chest X-ray. A sputum culture or blood tests may identify the organism, but in mild outpatient cases treatment often starts before the exact bug is known, guided by what is statistically most likely.
Why pneumonia makes you short of breath
Blood still flows past the flooded alveoli, but those alveoli hold pus instead of air. Blood leaves them just as oxygen-poor as it arrived — a shunt. The body responds by breathing faster (tachypnea) to wring more oxygen from the healthy lung. This is why a key warning sign of serious pneumonia is not just cough, but fast breathing and low oxygen saturation.
Reading a pulse oximeter in suspected pneumonia Normal at rest (room air): SpO2 95-100% Mild concern: SpO2 92-94% -> get assessed Serious, needs oxygen: SpO2 < 92% -> urgent care Why it matters: Flooded alveoli -> shunt -> oxygen-poor blood Body compensates -> breathing rate climbs (>20-24/min) Red-flag combination: SpO2 < 92% + respiratory rate > 24 = significant pneumonia until proven otherwise.