Pneumonia is an infection that inflames the tiny air sacs in your lungs, causing them to fill with fluid or pus. That buildup makes it harder for oxygen to reach your bloodstream, which is why even a mild case can leave you short of breath and exhausted. Globally, pneumonia kills a child every 39 seconds and an adult over 70 every 29 seconds, making it one of the deadliest infectious diseases on the planet.
What Happens Inside Your Lungs
Your lungs contain millions of small air sacs called alveoli. When you breathe in, oxygen passes through the thin walls of these sacs into your blood, and carbon dioxide passes out. Pneumonia disrupts this exchange.
When a germ reaches the alveoli, immune cells rush to the site and release chemical signals that trigger inflammation. That inflammatory response makes the tiny blood vessels in the lung walls “leaky,” allowing fluid to seep into the air sacs. The sacs that should be filled with air become waterlogged with fluid, pus, and dead cells. Oxygen can no longer cross efficiently into the blood, which is why pneumonia causes shortness of breath and low oxygen levels. In severe cases, the infection can spread from the lungs into the bloodstream, triggering a body-wide immune crisis known as sepsis.
Common Causes
Pneumonia isn’t caused by a single germ. Bacteria, viruses, and fungi can all trigger it, and the cause matters because it determines the treatment.
- Bacteria. The most common bacterial cause in the United States is Streptococcus pneumoniae (pneumococcus). Another bacterium, Mycoplasma pneumoniae, causes a milder form often called “walking pneumonia” because people feel well enough to stay on their feet. Legionella causes a severe type known as Legionnaires’ disease, typically linked to contaminated water systems.
- Viruses. Influenza and the common cold virus (rhinovirus) are the most frequent viral causes in adults. In young children, respiratory syncytial virus (RSV) is the leading viral culprit. SARS-CoV-2, the virus behind COVID-19, can also cause pneumonia.
- Fungi. Fungal pneumonia is less common and primarily affects people with weakened immune systems.
Symptoms to Recognize
Pneumonia symptoms can range from mild enough to confuse with a bad cold to severe enough to require hospitalization. The hallmark signs include a persistent cough (often producing thick mucus that may be yellow, green, or tinged with blood), fever and chills, shortness of breath during everyday activities, and chest pain that sharpens when you breathe deeply or cough.
Many people also experience fatigue, loss of appetite, nausea or vomiting, and confusion, particularly older adults. In fact, confusion may be the most prominent symptom in people over 65, sometimes appearing before any obvious respiratory signs. Infants and very young children may not cough at all but instead show rapid breathing, flared nostrils, or a bluish tint to the lips and fingernails.
Who Is Most at Risk
Age sits at the top of the risk ladder. Adults 65 and older face significantly higher risk, and that risk continues climbing with each additional year of age. On the other end, children under 5 are especially vulnerable, with the youngest toddlers and infants at greatest risk.
Chronic conditions amplify the danger. Heart disease, liver disease, chronic lung conditions like COPD or asthma, and diabetes all make it harder for the body to fight off lung infections. People with weakened immune systems, whether from medication, cancer treatment, or conditions like HIV, face the greatest risk of all.
Lifestyle factors play a role too. Smoking damages the airways’ natural defenses. Excessive alcohol use impairs the immune system. And simply spending time around people who are sick increases your exposure to the germs that cause pneumonia.
How Pneumonia Is Classified
Doctors categorize pneumonia by where you caught it, because the likely germs and the severity differ.
Community-acquired pneumonia (CAP) is the most common type. You pick it up in daily life, outside of a hospital. Hospital-acquired pneumonia (HAP) develops 48 hours or more after a hospital admission and tends to involve more drug-resistant bacteria. Ventilator-associated pneumonia (VAP) is a specific subset that develops more than 48 hours after a patient is placed on a breathing machine. HAP and VAP are generally more serious and harder to treat than CAP.
Diagnosis
A chest X-ray is the standard tool for confirming pneumonia. It shows whether fluid or inflammation has collected in the lungs and can reveal how much of the lung is affected. In hospital settings, lung ultrasound is sometimes used for rapid bedside assessment, especially if a patient is deteriorating quickly or the doctor needs to rule out other conditions like heart failure.
To gauge how serious a case is, doctors use scoring systems. One widely used tool assigns points for confusion, rapid breathing (30 breaths per minute or more), low blood pressure, elevated blood waste products, and age 65 or older. A higher score signals greater risk of complications and helps determine whether you can recover at home or need hospital care.
Treatment and Recovery Timeline
Bacterial pneumonia is treated with antibiotics. For most cases of community-acquired pneumonia, treatment lasts about 5 to 7 days. Major medical guidelines recommend a minimum of 5 days, with the expectation that you should be clinically stable (fever down, breathing improved, eating normally) before stopping. Hospital-acquired and ventilator-associated pneumonia typically require 7 to 8 days of treatment.
Viral pneumonia doesn’t respond to antibiotics. For flu-related pneumonia, antiviral medications can help if started early. COVID-related pneumonia may be treated with specific antivirals or anti-inflammatory therapies depending on severity. In many mild viral cases, treatment is supportive: rest, fluids, fever reducers, and time.
Recovery varies widely. Younger, otherwise healthy adults often feel significantly better within a week and return to normal activities within two to three weeks. Older adults and those with chronic conditions may take six weeks or longer to fully recover. Lingering fatigue is one of the most common complaints, sometimes persisting for a month or more even after the infection itself has cleared.
Potential Complications
The most common complication is fluid buildup between the lung and the chest wall, called a pleural effusion. Up to 44% of people hospitalized with pneumonia develop some degree of this fluid collection. Most of these effusions are simple and resolve on their own, but up to 30% become infected. When that happens, the risk of dying increases dramatically, by 3.4 to 7 times compared to pneumonia patients without effusions. Symptoms of an infected effusion include persistent or returning fevers and sharp chest pain that may radiate to the shoulder.
Pneumonia can also lead to bacteremia, where bacteria enter the bloodstream. From there, the infection can spread to other organs and trigger sepsis. Roughly 29% of patients with an infected pleural space show signs of sepsis in some studies. A lung abscess, a pocket of pus within the lung tissue, is another possible but less common complication.
Prevention Through Vaccination
Vaccination is the single most effective way to prevent the most common bacterial pneumonia. The CDC recommends routine pneumococcal vaccination for all children under 5, given as a four-dose series at 2, 4, 6, and 12 to 15 months of age.
For adults 50 and older who have never received a pneumococcal conjugate vaccine, the CDC recommends getting vaccinated. The newer vaccines (PCV20 and PCV21) are single-dose options that complete the vaccination in one shot. An older option (PCV15) requires a follow-up dose about a year later. Adults 65 and older who previously received older pneumococcal vaccines may benefit from an updated dose, a decision best made with their doctor based on their vaccination history.
Beyond pneumococcal vaccines, staying current on flu shots and COVID vaccines reduces the risk of viral pneumonia. Basic hygiene, including regular handwashing and avoiding close contact with sick individuals, provides an additional layer of protection.