Community-acquired pneumonia (CAP) is a lung infection you pick up in everyday life, outside of a hospital or other healthcare facility. That distinction matters because the germs responsible, and how they’re treated, differ from infections caught during a hospital stay. CAP is common: recent data show an overall incidence of about 42 per 1,000 people per year, with rates climbing sharply in people who have multiple chronic health conditions.
What Causes It
The single most common cause is a bacterium called Streptococcus pneumoniae, responsible for nearly half of all cases. Two other bacteria that behave a bit differently from typical germs, sometimes called “atypical” bacteria, also rank high on the list: one spread through respiratory droplets (Mycoplasma pneumoniae) and another linked to prolonged cough illness (Chlamydophila pneumoniae). Less frequently, other bacteria like Haemophilus influenzae and Staphylococcus aureus are involved.
Viruses cause a significant share of cases too. Influenza A is the most commonly identified viral culprit, though other respiratory viruses, including those that cause the common cold and flu-like illness, can also trigger pneumonia. In many cases, doctors never identify the exact germ. That’s partly because testing isn’t always done for mild cases, and partly because the infection may clear before a pathogen can be isolated.
Who Is Most at Risk
Certain chronic conditions raise your chances significantly. Heart disease, liver disease, chronic lung conditions like COPD or asthma, and diabetes all increase vulnerability. People with weakened immune systems, whether from medication, cancer treatment, or conditions like HIV, face the greatest risk. In people with two or more underlying conditions, CAP incidence can jump to over 160 per 1,000 per year, roughly four times the general rate.
Lifestyle factors play a role too. Smoking damages the airways’ natural defenses. Excessive alcohol use impairs the immune response and the cough reflex that helps keep the lungs clear. Simply being around people who are sick, especially in crowded settings, increases exposure to the germs that cause pneumonia.
Age is one of the strongest risk factors. Adults 65 and older are more likely to develop CAP and far more likely to be hospitalized or die from it.
Symptoms to Recognize
CAP typically comes on with a combination of respiratory and whole-body symptoms. You might experience a cough that produces mucus (sometimes rust-colored or greenish), chest pain that worsens when you breathe deeply, shortness of breath, and fever with chills. Fatigue, body aches, and loss of appetite are common. Some people, particularly older adults, may have confusion or a drop in alertness as one of the first noticeable signs, even before a cough develops.
“Atypical” pneumonia, often caused by Mycoplasma, tends to come on more gradually. It may feel more like a lingering cold with a dry cough and mild fever rather than the sudden, high-fever illness associated with bacterial pneumonia. Both forms still need medical attention.
How It’s Diagnosed
A chest X-ray is the standard tool for confirming pneumonia. It can reveal areas of consolidation, which are patches where the air sacs in your lungs have filled with fluid or pus instead of air. However, chest X-rays miss a meaningful number of cases. In one study comparing X-rays to CT scans, 27% of patients who had pneumonia visible on CT had a negative or inconclusive X-ray. If your symptoms strongly suggest pneumonia but the X-ray looks normal, your doctor may order a CT scan for a more detailed look.
Blood tests and sputum cultures can help identify the specific germ causing the infection, which guides treatment decisions. Pulse oximetry, a painless clip placed on your finger, measures your blood oxygen level and helps determine how severely the infection is affecting your breathing.
How Doctors Assess Severity
Not everyone with CAP needs to be in a hospital. Doctors use scoring tools to gauge how serious the infection is. One widely used system, called CURB-65, evaluates five factors: confusion, kidney function, breathing rate, blood pressure, and whether you’re 65 or older. Each factor gets one point. A score of 0 or 1 suggests the pneumonia is mild to moderate, and you may be treated at home. A score of 2 or higher points toward hospitalization.
The stakes at the severe end are real. Mortality rates can reach up to 50% in patients who develop serious complications like sepsis (a body-wide infection response) or respiratory failure. That’s why getting evaluated early matters, especially if you have underlying health conditions or are in an older age group.
Treatment and What to Expect
Bacterial CAP is treated with antibiotics. For otherwise healthy adults with mild pneumonia, treatment is typically an oral antibiotic taken at home for five to seven days. People with chronic conditions or more severe symptoms may need a different antibiotic combination or a course given through an IV in the hospital. Viral pneumonia doesn’t respond to antibiotics, though antiviral medication may be used if influenza is identified early enough.
Most people start feeling noticeably better within a few days of starting treatment, but full recovery takes longer than many expect. Some people return to normal routines in one to two weeks, while others need a month or more. Fatigue is especially persistent: most people continue to feel tired for about a month after the infection clears. If your symptoms haven’t improved after a reasonable stretch, your doctor may repeat imaging to check for other conditions that could be causing lingering problems.
Preventing CAP
Vaccination is the most effective preventive step. The CDC recommends pneumococcal vaccination for all adults 50 and older, as well as for adults 19 through 49 who have certain risk conditions like chronic lung or heart disease, diabetes, or immune system problems. Several vaccine options are available (PCV15, PCV20, and PCV21), and if PCV15 is used, a follow-up dose of a different pneumococcal vaccine is recommended to broaden protection.
Annual flu vaccination also reduces CAP risk, since influenza is a leading viral cause and can pave the way for a secondary bacterial pneumonia. Beyond vaccines, the basics still apply: frequent handwashing, avoiding close contact with people who are visibly ill, quitting smoking, and limiting alcohol intake all help keep your lungs’ natural defenses intact.