Community-acquired pneumonia (CAP) is a lung infection you pick up in everyday life, outside of a hospital or nursing facility. It’s one of the most common serious infections worldwide, sending roughly 1.2 million people to U.S. emergency departments each year and causing over 41,000 deaths annually. The term “community-acquired” simply distinguishes it from pneumonia caught during a hospital stay, which tends to involve different, often more resistant germs.
What Causes It
The two most frequently detected culprits are the bacterium Streptococcus pneumoniae (often called pneumococcus) and respiratory viruses. But in a surprisingly large share of cases, up to 62% in some hospital-based studies, no specific germ is ever identified despite thorough testing.
When a cause is found, it usually falls into one of three groups:
- Common bacteria: Pneumococcus is the single most common bacterial cause. Others include Haemophilus influenzae, Staphylococcus aureus, and Moraxella catarrhalis.
- Atypical bacteria: These are organisms that don’t show up on standard lab stains and don’t respond to penicillin-type antibiotics. Mycoplasma pneumoniae, Legionella, and Chlamydia pneumoniae are the main ones. Mycoplasma is sometimes associated with the milder illness people call “walking pneumonia.”
- Viruses: Influenza A and B, SARS-CoV-2, respiratory syncytial virus (RSV), rhinoviruses, and several others can all cause pneumonia on their own or set the stage for a bacterial infection on top.
Who Is Most at Risk
Age is one of the strongest risk factors. Adults 65 and older face higher rates of infection and more severe outcomes. Chronic conditions like heart disease, asthma, COPD, HIV, and cancer all raise susceptibility, as do smoking and heavy alcohol use. If you’ve had CAP before, your risk of getting it again is higher than average.
People with swallowing difficulties are at particular risk because food or saliva can slip into the lungs (aspiration), carrying bacteria with it. Sitting upright during meals, taking small bites, and maintaining good oral hygiene, including daily brushing and routine dental cleanings, can meaningfully lower that risk.
Symptoms to Recognize
CAP typically comes on over a few days with a combination of respiratory and whole-body symptoms. The hallmarks are cough (often producing mucus), fever or chills, shortness of breath, and chest pain that worsens when you breathe deeply or cough. Fatigue, body aches, and loss of appetite are common alongside these.
In older adults, the presentation can be subtler. Fever may be absent, and confusion or a general decline in function is sometimes the most noticeable sign. That’s part of why pneumonia in elderly patients is frequently diagnosed later and tends to be more dangerous.
How It’s Diagnosed
A chest X-ray is the standard tool for confirming pneumonia. When it shows a new area of cloudy, white density in the lung (called an infiltrate), that provides a definitive diagnosis. Doctors typically request views from both the front and the side, which improves accuracy and helps gauge severity.
In cases where the X-ray is unclear or complications are suspected, a CT scan of the chest gives a much more detailed picture. It can reveal cavities forming in the lung tissue, fluid collections around the lungs, or other problems that a plain X-ray might miss. Bedside lung ultrasound is also increasingly used, particularly in emergency settings, to look for areas of consolidated (solid-looking) lung tissue.
Gauging Severity
Not everyone with CAP needs to be hospitalized. Doctors use scoring tools to help decide who can safely recover at home and who needs closer monitoring. One widely used system, CURB-65, assigns one point each for five factors: confusion, elevated blood urea nitrogen (a kidney marker), rapid breathing (30 breaths per minute or more), low blood pressure, and age 65 or older.
The scores translate into strikingly different risk levels. A score of 0 or 1 carries about a 1.5% chance of dying within 30 days, and these patients are usually treated at home. A score of 2 bumps that risk to around 9%, often warranting hospital admission. A score of 3 or higher means roughly a 22% 30-day mortality rate, and intensive monitoring or ICU care may be needed.
Treatment and Recovery
For bacterial CAP, antibiotics are the cornerstone. If you’re otherwise healthy and being treated at home, the typical first-line option is a macrolide antibiotic (like azithromycin) or doxycycline. If you have chronic conditions such as heart disease, diabetes, liver or kidney problems, or have taken antibiotics recently, your doctor will likely choose a broader-spectrum antibiotic or a combination to cover resistant bacteria.
Current guidelines recommend a minimum of 5 days of antibiotic treatment for uncomplicated CAP. You should be fever-free for 48 to 72 hours and feeling meaningfully better before stopping. In practice, many people start to feel improvement within 2 to 3 days, but full recovery takes longer. Fatigue and a lingering cough can persist for several weeks even after the infection has cleared.
Viral pneumonia doesn’t respond to antibiotics. For influenza-related pneumonia, antiviral medications can shorten the illness if started early. For most other viral causes, treatment is supportive: rest, fluids, fever management, and supplemental oxygen if breathing becomes difficult.
Possible Complications
The most common complication is fluid buildup between the lung and the chest wall, called a parapneumonic effusion. This occurs in about 35 to 40% of patients with bacterial pneumonia, and the rate climbs to nearly 60% in cases caused by pneumococcus specifically. Viral and mycoplasma pneumonia cause smaller effusions in about 20% of cases.
If that fluid becomes infected, it turns into empyema, a pocket of pus in the chest cavity. Bacterial pneumonia accounts for about 70% of empyema cases. Roughly 15 to 25% of patients with empyema ultimately need a surgical procedure to drain the infected fluid or remove a thick rind of scar tissue from the lung surface. Other possible complications include lung abscesses, bacteria spreading into the bloodstream, and inflammation affecting other organs.
Prevention Through Vaccination
Vaccination is the most effective way to reduce your risk of CAP and its severity. The CDC currently recommends pneumococcal vaccination for all adults 50 and older who haven’t previously received a pneumococcal conjugate vaccine. Three conjugate vaccine options are available (PCV15, PCV20, and PCV21). If PCV20 or PCV21 is used, no additional pneumococcal vaccination is needed. If PCV15 is used, a follow-up dose of a different pneumococcal vaccine (PPSV23) is recommended about a year later.
Beyond pneumococcal vaccines, staying current on influenza, COVID-19, and RSV vaccinations also helps prevent pneumonia, particularly for older adults and those with chronic health conditions. For people who’ve already had an episode of CAP, quitting smoking, reducing alcohol intake, and keeping up with all relevant vaccinations are the most impactful steps to avoid a recurrence.