You get pneumonia when a germ, most often a bacterium or virus, reaches the tiny air sacs deep in your lungs and triggers an inflammatory response that fills those sacs with fluid. That fluid is what makes breathing difficult and produces the hallmark symptoms: cough, fever, and shortness of breath. In the United States alone, pneumonia sends about 1.2 million people to the emergency department each year and causes more than 41,000 deaths.
What Happens Inside Your Lungs
Your lungs contain millions of tiny air sacs called alveoli, where oxygen passes into your blood and carbon dioxide passes out. Under normal conditions, these sacs stay open and dry. When a pathogen makes it past your body’s defenses and reaches the alveoli, your immune system launches an aggressive counterattack. White blood cells called neutrophils flood out of nearby blood vessels and into the air sacs, releasing toxic chemicals designed to kill the invader.
That battle produces collateral damage. The alveoli fill with a mix of fluid, dead cells, proteins, and debris. This is the “consolidation” that shows up as white patches on a chest X-ray. With the air sacs clogged, oxygen can no longer cross into your bloodstream efficiently, which is why pneumonia makes you feel short of breath even at rest. The worse the infection, the more air sacs fill up, and the harder your lungs have to work.
How Germs Get Into Your Lungs
Most people assume pneumonia starts with breathing in someone else’s germs, and that does happen. But the most common route is actually microaspiration: tiny amounts of saliva or mucus from your mouth and throat slip into your lungs, usually during sleep. Your mouth is home to hundreds of bacterial species, and normally your cough reflex and immune defenses clear these small intrusions before they cause problems. Pneumonia develops when those defenses are overwhelmed or compromised.
Larger-scale aspiration is another pathway. If food, liquid, stomach acid, or vomit enters your airway, it can carry bacteria directly into the lungs. This is called aspiration pneumonia, and it’s especially common in people who have trouble swallowing after a stroke or brain injury, those with neurological conditions like Parkinson’s disease, people recovering from general anesthesia, and anyone impaired by alcohol or drugs.
Bacteria, Viruses, and Fungi
Bacterial pneumonia is the type most people picture. The single most common culprit is Streptococcus pneumoniae, a bacterium that spreads through direct contact with respiratory secretions like saliva or mucus. It can live harmlessly in your throat until something, like a cold or flu, weakens your defenses enough to let it move deeper into the lungs. Bacterial pneumonia tends to come on suddenly with a high fever and thick, colored mucus.
Viruses cause a significant share of pneumonia cases, particularly in young children and older adults. Influenza, RSV (respiratory syncytial virus), and SARS-CoV-2 can all infect the lower respiratory tract. RSV is the most common cause of pneumonia in infants. Viral pneumonia often starts gradually, with symptoms that feel like a bad cold before progressing to the lungs. In some cases, a viral infection damages the airways enough to allow bacteria to move in on top of it, creating a secondary bacterial pneumonia that can be more severe than either infection alone.
Fungal pneumonia is less common and typically affects people with weakened immune systems. Certain fungi live in soil or bird droppings in specific regions, and inhaling their spores can cause lung infections that healthy immune systems usually handle without symptoms.
When Your Immune System Makes Things Worse
Paradoxically, much of the damage in severe pneumonia comes not from the germ itself but from your body’s own response. Research on severe COVID-19 pneumonia at Columbia University illustrated this clearly: signaling molecules called cytokines, released by immune cells in the lungs, created a kind of chain reaction that pulled even more immune cells from the bloodstream into the airways. These cells, called monocytes, normally never reach the air sacs, but in severe cases they accumulated throughout the lungs and clogged the alveolar spaces.
This overreaction isn’t unique to COVID. In any severe pneumonia, the immune system can essentially flood the battlefield, destroying lung tissue along with the pathogen. The inflammation causes the alveoli to leak more fluid, which attracts more immune cells, which release more inflammatory signals. Breaking that cycle is one reason why anti-inflammatory treatments sometimes help in severe pneumonia, even though the underlying problem is an infection.
Who Is Most Vulnerable
Age is the single biggest risk factor, and it works at both ends of life. Adults 65 and older face significantly higher risk, and that risk keeps climbing with each additional year: an 80-year-old is at greater risk than a 65-year-old. Children under 5 are also more vulnerable, with the youngest at highest risk.
Chronic health conditions amplify the danger considerably. Heart disease, liver disease, lung disease (including asthma and COPD), and diabetes all increase susceptibility. People with weakened immune systems, whether from disease, chemotherapy, or medications that suppress immunity, face the greatest risk of all.
Lifestyle factors matter too. Smoking damages the cilia, the tiny hair-like structures that sweep debris and germs out of your airways, making it easier for pathogens to settle in the lungs. Excessive alcohol use impairs the cough reflex and weakens immune function. Even something as simple as spending a lot of time around sick people raises your odds, which is why pneumonia spreads easily in nursing homes, hospitals, and daycare centers.
Community-Acquired vs. Hospital-Acquired Pneumonia
Pneumonia picked up in everyday life is called community-acquired pneumonia, and it’s usually caused by common bacteria like Streptococcus pneumoniae or respiratory viruses. Most cases can be treated at home with rest and, if bacterial, a course of antibiotics.
Hospital-acquired pneumonia is a different problem. It develops 48 hours or more after a hospital admission, often in patients who are already weakened by another illness or surgery. The bacteria responsible tend to be tougher organisms, including drug-resistant strains like MRSA, that thrive in hospital environments. Patients on ventilators are particularly vulnerable because the breathing tube bypasses the throat’s natural defenses and provides a direct path for bacteria to reach the lungs. Hospital-acquired pneumonia is generally harder to treat and carries a higher mortality rate than the community-acquired type.
Vaccination and Prevention
Pneumococcal vaccines target the most common bacterial cause of pneumonia. Despite their availability, only about 25% of U.S. adults have ever received one. Newer vaccines cover a broader range of bacterial strains, though long-term efficacy data on the latest formulations is still being collected. Annual flu and COVID vaccines also reduce pneumonia risk indirectly by preventing the viral infections that often precede it.
Beyond vaccination, the most effective prevention strategies are straightforward: regular handwashing, not smoking, limiting alcohol intake, and managing chronic conditions like diabetes and heart disease. For older adults or anyone with swallowing difficulties, eating slowly, staying upright during and after meals, and maintaining good oral hygiene can reduce the risk of aspiration pneumonia.