Pneumonia is an infection that inflames the air sacs in your lungs, causing them to fill with fluid or pus. This makes it harder for oxygen to reach your bloodstream, which is why breathing feels difficult and your body feels so drained. It ranges from mild enough to treat at home to severe enough to require hospitalization. In the United States, pneumonia sends about 1.2 million people to emergency departments each year and causes over 41,000 deaths annually.
What Happens Inside Your Lungs
Your lungs contain millions of tiny air sacs that handle the exchange of oxygen and carbon dioxide with every breath. When a pathogen like a bacterium or virus reaches these air sacs, immune cells rush to the site and release chemical signals that recruit more defenders. This immune response is what actually causes most of the damage you feel. The walls of tiny blood vessels in the lungs become “leaky,” allowing fluid, infection-fighting cells, and debris to flood into the air sacs.
In the early stage, the lung tissue becomes heavy and waterlogged with fluid rich in infectious organisms. As the infection progresses, red blood cells and clotting proteins pour into the air sacs too, making affected areas of the lung dense and solid. This is why breathing becomes painful and labored: portions of your lung that should be filled with air are instead packed with fluid and inflammatory material, dramatically reducing how much oxygen your body can absorb.
Common Causes
Bacteria are the most frequent culprits. The single most common cause of community-acquired pneumonia is a bacterium called Streptococcus pneumoniae. Other bacterial causes include Haemophilus influenzae and the organism responsible for Legionnaires’ disease.
Viruses that cause the common cold, flu, COVID-19, and RSV can also lead to pneumonia. Viral pneumonia tends to produce a dry, nonproductive cough rather than the thick mucus associated with bacterial infections, though the two can overlap. Fungal pneumonia is less common and typically affects people with weakened immune systems.
Where you pick up the infection matters. Pneumonia caught in everyday life (community-acquired) is generally easier to treat than pneumonia acquired in a hospital, which is more likely to involve antibiotic-resistant bacteria like MRSA. People on mechanical ventilators in intensive care face a distinct risk category as well, since the breathing tube can introduce pathogens directly into the lungs.
Symptoms to Recognize
Fever and cough are nearly universal. In studies of influenza-related pneumonia, shortness of breath occurs in about 54% of patients, fatigue or weakness in 40%, chills in 37%, and muscle aches in 36%. Sore throat, headache, and vomiting each appear in roughly 30% of cases. With other viral types, cough shows up in about 90% of patients and difficulty breathing in over 80%.
The character of the cough varies by cause. Bacterial pneumonia often produces thick yellow, green, or even blood-tinged mucus. Viral pneumonia more commonly starts with a dry cough that may later become productive. In older adults, cough can be minimal or absent, and confusion or a sudden decline in daily functioning may be the most obvious sign that something is wrong.
How Pneumonia Is Diagnosed
A chest X-ray is the standard first step. It reveals areas of inflammation in the lungs, appearing as white or hazy patches where healthy lung tissue should look dark. If your doctor needs more detail, or suspects complications like a lung abscess or fluid collecting around the lung, a CT scan provides a much more detailed picture. Blood tests, particularly a complete blood count, help confirm whether your immune system is actively fighting an infection and can offer clues about whether the cause is bacterial or viral.
Who Faces the Greatest Risk
Age sits at both extremes of the risk spectrum. Children under 4 account for about 70% of complicated pneumonia cases, and adults 65 and older face significantly higher rates of hospitalization and death. Beyond age, the people most vulnerable include those with chronic lung conditions, heart disease, weakened immune systems, neuromuscular disorders that affect the ability to cough or swallow, and anyone with a decreased level of consciousness that raises the chance of inhaling food or saliva into the lungs.
Doctors assess severity using a scoring system that considers five factors: confusion, kidney function, breathing rate, blood pressure, and whether the patient is 65 or older. A score of 0 or 1 suggests the infection can likely be managed at home. A score of 2 or higher points toward hospitalization.
Possible Complications
Most otherwise healthy people recover from pneumonia without lasting problems. But when the infection is severe or the person’s defenses are compromised, serious complications can develop. Fluid can accumulate in the space between the lung and chest wall, sometimes becoming infected itself and forming a pocket of pus called an empyema. In other cases, the infection destroys patches of lung tissue, creating abscesses or air-filled cavities.
The most dangerous complication is when the infection spills into the bloodstream, triggering a bodywide inflammatory response. This can cause blood pressure to plummet and organs to fail. Premature infants, people with chronic lung disease, and those with suppressed immune systems carry the highest risk for these outcomes.
Recovery Timeline
Some people feel better and return to their normal routine within 1 to 2 weeks. For others, recovery takes a month or longer. Even after the infection clears, most people continue to feel tired for about a month. This lingering fatigue catches many people off guard, especially if the acute symptoms resolved quickly. Pushing back to full activity too soon can slow the process, so a gradual return to normal exertion is typical.
Older adults and those who were hospitalized generally face a longer recovery arc. It’s not unusual for full lung function to take several months to return in severe cases.
Prevention and Vaccines
Pneumococcal vaccines target the most common bacterial cause of pneumonia. The CDC recommends pneumococcal vaccination for all adults 50 and older, as well as adults 19 through 49 who have certain risk conditions like chronic heart or lung disease, diabetes, or a weakened immune system. Several vaccine formulations are available, and the choice depends on what you’ve received previously.
Annual flu and COVID-19 vaccines also reduce pneumonia risk indirectly, since these viral infections are common triggers. Beyond vaccination, basic measures like frequent handwashing, not smoking, and managing chronic health conditions all lower your chances of developing a lung infection in the first place.