Pneumonia is an infection of one or both lungs that fills the tiny air sacs with fluid or pus. It is usually caused by bacteria or viruses, sometimes by fungi. Typical symptoms are a cough (often with phlegm), fever or chills, shortness of breath and chest pain when you breathe or cough. Older adults may only show confusion, weakness or a low temperature.
Most people recover at home with the right treatment and feel close to normal within about two weeks. But pneumonia can be serious: it caused 41,627 deaths in the United States in 2024, according to the CDC. Doctors use simple tools to tell who is at risk: a chest X-ray, an oxygen reading, a few blood tests and a short severity score.
This guide explains how pneumonia is diagnosed, what your blood results mean (white cells, CRP, procalcitonin, BUN), whether it is contagious, what "walking pneumonia" is, and what changed in the 2025 guideline.
Key takeaways
- Pneumonia is a lung infection with many possible germs: pneumococcus, Mycoplasma, flu, COVID-19 and RSV are common.
- Diagnosis rests on symptoms, a lung exam and a chest X-ray. Blood tests show how serious it is and whether bacteria are likely.
- Scores such as CURB-65 help decide between home care, a hospital ward or intensive care.
- Since the 2025 ATS guideline, many adults who improve quickly can stop antibiotics after 3 to 5 days.
- Vaccines lower your risk: pneumococcal vaccine from age 50, flu and COVID-19 vaccines, and RSV vaccine for older adults.
What is pneumonia?
Your lungs end in millions of small air sacs called alveoli, where oxygen passes into the blood. In pneumonia, an infection inflames these sacs and they fill with fluid. Less air gets through, so you breathe faster, cough and your oxygen level can drop.
Doctors classify pneumonia by where you caught it, because this changes the likely germs:
- Community-acquired pneumonia (CAP): caught in daily life. This is the most common type and the focus of this guide.
- Healthcare-acquired pneumonia: develops during or after a hospital or care-facility stay.
- Ventilator-associated pneumonia: develops in someone on a breathing machine.
- Aspiration pneumonia: caused by inhaling food, drink or saliva, often in people who have trouble swallowing.
If your main concern is pain in the back or side when you breathe, our guide to pneumonia and back pain explains when that pain points to the lungs.
Causes and risk factors
The CDC lists the main causes in the United States:
- Bacteria: mostly Streptococcus pneumoniae (pneumococcus) and Mycoplasma pneumoniae; less often Haemophilus influenzae, Legionella and Chlamydia pneumoniae.
- Viruses: influenza, COVID-19, RSV, human metapneumovirus, parainfluenza and rhinovirus. Our article on influenza symptoms and treatment explains how flu can lead to pneumonia.
- Fungi, rarer and mostly in people with weak immunity.
Often no germ is found, even after testing. This does not mean the treatment is wrong. According to the CDC and MedlinePlus, the main risk factors are:
- Age: adults 65 and older, and children under 5.
- A weakened immune system, which carries the greatest risk.
- Chronic heart, lung (COPD, asthma), liver or kidney disease, and diabetes.
- Smoking, heavy drinking, contact with sick people, and the fall and winter season.
Symptoms and warning signs
Symptoms can start suddenly or build up over a few days:
- Cough, often with yellow, green, rust-colored or bloody mucus.
- Fever, sweating and shaking chills.
- Shortness of breath and fast breathing.
- Sharp chest pain that worsens when you breathe in or cough.
- Tiredness, loss of appetite, and sometimes nausea, vomiting or diarrhea.
In older adults and people with weak immunity, symptoms can be surprisingly mild: confusion, weakness or a low temperature may be the only clue. Urgent warning signs include bluish lips, trouble breathing at rest, confusion and symptoms that get worse after first improving.
Walking pneumonia
"Walking pneumonia" is the common name for pneumonia caused by Mycoplasma pneumoniae. The CDC explains that people often feel well enough to keep going to work or school. It causes a cough that can last weeks, tiredness, fever and shortness of breath; young children may have a runny nose or wheezing instead. Symptoms appear 1 to 4 weeks after exposure. Mycoplasma is the most common bacterial cause of pneumonia in hospitalized children and the second most common in adults.
Is pneumonia contagious?
The germs that cause pneumonia are often contagious. Flu, COVID-19, RSV, pneumococcus and Mycoplasma spread through coughs, sneezes and close contact. One exposed person may get only a cold while another, often older or chronically ill, develops pneumonia. Aspiration pneumonia is not contagious.
The CDC advises staying away from others while you are sick, covering coughs, washing your hands and cleaning shared surfaces. Ask your doctor when you can return to work or school.
Diagnosis and tests
There is no single "pneumonia blood test". The diagnosis combines symptoms, a physical exam and imaging. Other tests answer three questions: how sick are you, is the cause bacterial or viral, and is home treatment safe?
Exam and pulse oximetry
Your doctor listens for crackles and checks temperature, heart rate, breathing rate and blood pressure. A finger pulse oximeter measures oxygen saturation (SpO2). MedlinePlus states that normal is 95% to 100%; 92% or lower is a reason to contact your provider, and 88% or lower needs immediate care. Readings can be off by 2% to 4% and tend to overestimate oxygen in darker skin. If you feel very short of breath despite a normal number, an arterial blood gas gives a precise value.
Imaging
A chest X-ray is usually the first test. It shows the infected area and complications such as fluid around the lung. The 2025 ATS guideline also supports lung ultrasound by experienced clinicians. CT is kept for unclear cases. In smokers, a pneumonia that does not clear should be checked again; see our guide to lung cancer diagnosis.
Blood tests
- Complete blood count (CBC): a high white cell count, mainly neutrophils, suggests bacteria; a very low count can signal severe infection. Our complete blood count guide explains each line.
- C-reactive protein (CRP): a general marker of inflammation that also tracks recovery.
- Procalcitonin: rises more in bacterial than in viral infections.
- BUN, creatinine and sodium: a high BUN is a CURB-65 danger point, and low sodium is common in severe pneumonia.
- Blood cultures: recommended for severe pneumonia and for people at risk of resistant germs, not for everyone.
Sputum, swabs and urine
- Sputum culture: mainly for hospitalized patients with severe pneumonia.
- Nose or throat swab: rapid molecular (PCR) tests for flu, COVID-19, RSV and sometimes Mycoplasma.
- Urinary antigen tests for pneumococcus and Legionella: suggested in severe pneumonia, and for Legionella during outbreaks or after travel.
With a long cough, weight loss and night sweats, your doctor may also check for tuberculosis; see our tuberculosis test guide.
Reference values and severity thresholds
These are the numbers doctors look at. Normal ranges vary slightly between laboratories, so compare with the range on your own report.
| Test or sign | Usual value (US / SI) | Threshold of concern |
|---|---|---|
| Oxygen saturation (SpO2) | 95% to 100% | 92% or less: call your provider; 88% or less: emergency |
| White blood cells | 4,500 to 11,000/µL (4.5 to 11.0 × 10⁹/L) | Below 4,000/µL (4.0 × 10⁹/L) from infection: severe-pneumonia criterion |
| Platelets | 150,000 to 400,000/µL (150 to 400 × 10⁹/L) | Below 100,000/µL (100 × 10⁹/L): severe-pneumonia criterion |
| Blood urea nitrogen (BUN) | About 7 to 20 mg/dL (2.5 to 7.1 mmol/L) | Urea above 7 mmol/L (about 19 mg/dL): 1 CURB-65 point; 30 mg/dL (11 mmol/L) or more: PSI points |
| Sodium | 135 to 145 mEq/L (mmol/L) | Below 130 mEq/L: PSI points |
| Arterial pH | 7.35 to 7.45 | Below 7.35: PSI points |
| Procalcitonin | Low in healthy people (ng/mL = µg/L) | 0.1 µg/L or less: bacteria less likely; 0.25 µg/L or more: more likely (study cutoffs, never used alone) |
| Breathing rate (adults) | About 12 to 20 per minute | 30 or more: 1 CURB-65 point |
| Blood pressure | Below 120/80 mm Hg | Systolic below 90 or diastolic 60 or less: 1 CURB-65 point |
| Temperature | About 97 to 99 °F (36.1 to 37.2 °C) | Below 96.8 °F (36 °C): severe-pneumonia criterion |
How to read your results
CURB-65
CURB-65 gives one point each for Confusion, Urea above 7 mmol/L, Respiratory rate of 30 or more, low Blood pressure and age 65 or older. In the study that created it, 30-day mortality was 0.7% with a score of 0, 3.2% with 1, 3% with 2, 17% with 3, 41.5% with 4 and 57% with 5. A score of 0 or 1 usually allows home treatment; 3 or more points to hospital care.
The Pneumonia Severity Index
The PSI adds points for age, other diseases, vital signs and lab values such as BUN, sodium and blood pH. The ATS/IDSA guideline prefers it, with CURB-65 as an alternative, always alongside the doctor's judgment. Someone who cannot keep fluids down or has very low oxygen may need the hospital even with a "low" score.
White cells, CRP and procalcitonin together
No single marker proves bacterial pneumonia. A high white blood cell count with many neutrophils, a clearly raised CRP and a procalcitonin of 0.25 µg/L or more all point toward bacteria. Normal values make a virus more likely but do not exclude bacteria, especially early. Our guides to high neutrophils and the CRP blood test go further.
The ATS/IDSA guideline says antibiotics should start when pneumonia is confirmed, whatever the first procalcitonin result. The test is most useful later, to decide when to stop. In a pooled analysis of 26 trials and 6,708 patients, procalcitonin guidance cut antibiotic use by 2.4 days, reduced side effects and slightly lowered mortality. See our page on the procalcitonin test.
A rising BUN can mean dehydration or kidney strain; our BUN blood test guide explains why. If infection spreads to the blood, the concern becomes sepsis, covered in our article on the sepsis blood test.
Treatment and follow-up
Antibiotics
For healthy adults treated at home, the ATS/IDSA guideline lists amoxicillin (a penicillin that blocks the bacterial cell wall), doxycycline, or a macrolide such as azithromycin only where local pneumococcal resistance is below 25%. People with heart, lung, liver or kidney disease, diabetes or cancer usually get amoxicillin-clavulanate or a cephalosporin plus a macrolide or doxycycline, or a fluoroquinolone such as levofloxacin, which blocks bacterial DNA gyrase. In the hospital, the usual choice is a beta-lactam such as ceftriaxone plus azithromycin, or a fluoroquinolone alone.
How long?
Length now depends on how fast you improve. "Clinical stability" means normal vital signs, eating and normal alertness. The 2025 ATS guideline recommends less than 5 days (at least 3) for nonsevere pneumonia once stable, and 5 days or more for severe pneumonia. Never stop on your own: follow your doctor's plan.
Viral pneumonia and hospital care
Antibiotics do not work on viruses. Flu pneumonia is treated with an antiviral such as oseltamivir, which blocks the virus's neuraminidase enzyme; COVID-19 has its own antivirals. Hospital care may include oxygen, fluids and breathing support. Steroids such as hydrocortisone may be used in severe pneumonia, not in nonsevere cases. At home, rest, drink fluids and use fever medicine if your doctor agrees.
Recovery
With treatment, most people feel nearly normal within about 2 weeks, though tiredness and cough can last a month or longer. A follow-up X-ray is not routine if you recover well, but may be ordered if symptoms persist or you smoke. The X-ray often takes weeks to clear, longer than your symptoms.
Prevention
- Pneumococcal vaccine: since October 2024, CDC advisers recommend a conjugate vaccine (PCV20 or PCV21 alone, or PCV15 then PPSV23) for all adults 50 and older and for adults 19 to 49 with risk conditions. Children get it routinely. Only 25.8% of US adults had ever received one in 2025.
- Yearly flu vaccine; see our article on the 2026 flu shot.
- RSV vaccine for all adults 75 and older and for adults 50 to 74 at increased risk.
- COVID-19, whooping cough, Hib and measles vaccines as recommended for your age.
- Hand washing, covering coughs, not smoking and keeping chronic diseases controlled.
Pneumonia in children, pregnancy and older adults
Children
The WHO advises suspecting pneumonia in a child under 5 with cough or trouble breathing, and confirming it by fast breathing or chest indrawing, when the lower chest pulls in as the child breathes in. Inability to drink, unusual sleepiness or convulsions are danger signs. Amoxicillin is the first-line antibiotic. In the US SCOUT-CAP trial, 5 days of antibiotics worked as well as 10 days in improving children aged 6 months to 5 years.
Pregnancy
Flu and COVID-19 can be more severe during pregnancy, so vaccination matters. An RSV vaccine given in pregnancy protects the newborn; otherwise infants can get an antibody shot (nirsevimab or clesrovimab). Our guide to blood tests during pregnancy explains routine checks.
Older adults
People 65 and older are more likely to need hospital care and to have complications. Symptoms are often atypical: confusion, a fall or poor appetite instead of high fever. Immune changes with age are discussed in our article on lymphocytes and aging.
When to see a doctor
See a doctor the same day for a cough with fever, chills or shortness of breath, chest pain when you breathe, or rust-colored or bloody mucus. Act sooner if you are 65 or older, pregnant, have a chronic disease or weak immunity, or if your child breathes fast.
Call 911 or go to the emergency room for:
- Severe trouble breathing, or a chest that pulls in with each breath.
- Bluish lips or fingertips, or an oximeter reading of 88% or lower.
- Confusion, or being very drowsy or hard to wake.
- Severe chest pain, a very fast heartbeat or fainting.
- Getting worse on antibiotics, or improving then suddenly worsening.
Latest scientific advances in pneumonia
The 2025 ATS guideline: shorter courses and a debate
The American Thoracic Society updated its adult guideline in 2025 (published January 2026). It supports lung ultrasound, recommends 3 to under 5 days of antibiotics for stable nonsevere pneumonia, and suggests antibiotics for patients with a positive virus test if they have other diseases or are hospitalized. The Infectious Diseases Society of America agreed with 8 of 10 recommendations but did not endorse the update over that last point: most patients with a positive virus test have no bacteria, and briefly waiting is safe in nonsevere cases. For you: shorter courses are now standard, and a positive flu or COVID-19 test does not settle the antibiotic question on its own.
Steroids in severe pneumonia
In the French CAPE COD trial (NCT02517489), 800 intensive-care patients received hydrocortisone or placebo; 28-day deaths fell from 11.9% to 6.2%. But REMAP-CAP (NCT02735707), published in 2025, found no clear benefit of a fixed 7-day course on 90-day mortality. Steroids remain a hospital decision for severe cases only.
Pneumococcal vaccine from age 50
In June 2024, the FDA approved PCV21 (Capvaxive), which adds eight serotypes not in earlier vaccines. In October 2024, CDC advisers lowered the age for routine pneumococcal vaccination from 65 to 50. If you are 50 or older and never had a conjugate vaccine, one dose is now recommended.
The return of walking pneumonia
Mycoplasma infections dropped during the COVID-19 pandemic, then surged. In 42 US children's hospitals, Mycoplasma pneumonia reached 12.5 per 1,000 hospitalizations in 2024, versus 2.1 in 2018 to 2023, mostly in children aged 6 to 12; cases were not more severe. In Ohio, macrolide resistance was low (2.4%) but rising. A Mayo Clinic trial (NCT07164131) now compares azithromycin with doxycycline in hospitalized adults.
Pulse oximeters and skin color
A 2026 review confirms that finger oximeters can overestimate oxygen in darker skin, especially when oxygen is already low. At home, watch the trend and how you feel, not only the number.
Which tests can help?
After a visit or a hospital stay, your report often lists a CBC, CRP, procalcitonin and a metabolic panel. Knowing them helps you follow recovery:
- CBC: white cells and neutrophils should return to normal as you recover. Our comparison of the CBC and CMP explains each panel.
- CRP: falls within days when treatment works; a CRP that does not fall, with persistent fever, is a reason to call your doctor.
- Procalcitonin: a falling level supports stopping antibiotics.
- BUN, creatinine and sodium: check hydration and kidney function.
- A rapid test to separate bacterial from viral infection is coming; read about the 15-minute bacterial or viral blood test.
If a value is flagged, our guide to abnormal blood test results explains what usually matters.
Glossary
- Alveoli: tiny air sacs where oxygen enters the blood.
- CURB-65: a five-point severity score.
- Pneumonia Severity Index (PSI): a longer score using age, diseases, vital signs and lab results.
- Procalcitonin: a blood protein that rises mostly in bacterial infections.
- CRP: a liver-made marker of inflammation.
- Pulse oximetry: a finger clip that estimates oxygen saturation.
- Urinary antigen test: detects pieces of pneumococcus or Legionella in urine.
- Sepsis: a life-threatening body reaction to infection that damages organs.
- Walking pneumonia: mild pneumonia, usually from Mycoplasma pneumoniae.
Frequently asked questions
Is pneumonia contagious?
The germs behind most pneumonia, such as flu, COVID-19, RSV, pneumococcus and Mycoplasma, spread through coughs and sneezes, but not everyone who catches them gets pneumonia. Aspiration pneumonia is not contagious. Stay away from others while you have a fever.
What is the prognosis for pneumonia?
Most people recover fully, many within about 2 weeks, though fatigue can last a month or more. Risk rises with age and other diseases: in the CURB-65 study, 30-day mortality was under 1% with a score of 0 but 57% with a score of 5.
Can you swim with pneumonia?
Better not while you have a fever, shortness of breath or a heavy cough. Swimming is demanding on the lungs and you could spread the germ. Wait until you feel well and breathe normally, then build up slowly, and ask your doctor after a hospital stay.
What are the different types of pneumonia that affect elderly people?
Mostly community-acquired pneumonia from pneumococcus, flu, RSV or COVID-19; aspiration pneumonia in people with swallowing problems; and healthcare-acquired pneumonia in hospitals and nursing homes. Symptoms can be as subtle as confusion or a fall.
What are the different stages of pneumonia in children?
Doctors grade it by severity signs: first cough or trouble breathing with fast breathing; then chest indrawing; then danger signs such as inability to drink, unusual sleepiness or convulsions. Each step needs faster care, up to the emergency room.
What causes pneumonia in adults?
Mostly pneumococcus, respiratory viruses and Mycoplasma. Smoking, heavy drinking, chronic diseases and weak immunity raise the risk.
How can you prevent pneumonia?
Get the pneumococcal vaccine from age 50 (earlier if at risk), a yearly flu shot, COVID-19 and RSV vaccines if eligible, wash your hands and avoid smoking.
What precautions are taken for pneumonia in the hospital?
Hospitals use hand hygiene, masks and isolation for contagious germs such as flu or COVID-19. Visitors should clean their hands and stay home when sick.
Sources
- Jones BE et al. — Diagnosis and Management of Community-acquired Pneumonia: An Official ATS Clinical Practice Guideline — Am J Respir Crit Care Med, 2026 — doi.org
- Metlay JP et al. — Diagnosis and Treatment of Adults with Community-acquired Pneumonia (ATS/IDSA) — Am J Respir Crit Care Med, 2019 — doi.org
- Klompas M et al. — Why IDSA Did Not Endorse the Community-Acquired Pneumonia Guidelines 2025 Update — Clin Infect Dis, 2026 — doi.org
- Long B, Gottlieb M — 2025 guideline updates for community-acquired pneumonia — Am J Emerg Med, 2026 — doi.org
- Dequin PF et al. — Hydrocortisone in Severe Community-Acquired Pneumonia — N Engl J Med, 2023 — doi.org
- REMAP-CAP Investigators — Effect of hydrocortisone on mortality in severe community-acquired pneumonia — Intensive Care Med, 2025 — doi.org
- Lim WS et al. — Defining community acquired pneumonia severity on presentation to hospital — Thorax, 2003 — pubmed.ncbi.nlm.nih.gov
- Fine MJ et al. — A prediction rule to identify low-risk patients with community-acquired pneumonia — N Engl J Med, 1997 — pubmed.ncbi.nlm.nih.gov
- Schuetz P et al. — Procalcitonin-guided antibiotic treatment and mortality in acute respiratory infections: a patient level meta-analysis — Lancet Infect Dis, 2018 — doi.org
- Williams DJ et al. — Short- vs Standard-Course Outpatient Antibiotic Therapy for Pneumonia in Children: SCOUT-CAP — JAMA Pediatr, 2022 — doi.org
- Kobayashi M et al. — Expanded Recommendations for Pneumococcal Conjugate Vaccines Among Adults Aged ≥50 Years (ACIP) — MMWR, 2025 — doi.org
- Kobayashi M et al. — Use of 21-Valent Pneumococcal Conjugate Vaccine Among U.S. Adults (ACIP) — MMWR, 2024 — doi.org
- Diaz MH et al. — Mycoplasma pneumoniae Infections in Hospitalized Children, United States, 2018-2024 — MMWR, 2025 — doi.org
- Macrolide-Resistant Mycoplasma pneumoniae Infections among Children after COVID-19 Pandemic, Ohio — Emerg Infect Dis, 2025 — doi.org
- Savorgnan F et al. — Pulse Oximetry, Skin Pigmentation, and Occult Hypoxemia — Pediatr Cardiol, 2026 — doi.org
- Moulia DL et al. — Use of Clesrovimab for Prevention of Severe RSV Infections in Infants (ACIP) — MMWR, 2025 — doi.org
- Mayo Clinic — Azithromycin Versus Doxycycline in Hospitalized Adults With Community Acquired Pneumonia (NCT07164131) — ClinicalTrials.gov, 2025 — clinicaltrials.gov
- CDC — About Pneumonia; Risk Factors; Prevention — CDC, 2025 — cdc.gov
- CDC National Center for Health Statistics — FastStats: Pneumonia — CDC, 2026 — cdc.gov
- CDC — About Mycoplasma pneumoniae Infection — CDC, 2025 — cdc.gov
- CDC — Pneumococcal Vaccines; RSV Vaccines — CDC, 2025-2026 — cdc.gov
- MedlinePlus — Community-acquired pneumonia in adults; Pulse Oximetry; WBC count — NLM, 2025 — medlineplus.gov
- World Health Organization — Pneumonia in children (fact sheet) — WHO, 2022 — who.int
Further reading
- Pneumonia and back pain: causes and red flags
- High CRP levels: what your result really means
- Low absolute neutrophils: causes and treatment
- Sodium blood test: normal, low and high levels
- mRNA flu vaccine: what the first approval means
- Is a sinus infection contagious?
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