Pneumonia in adults is caused by infections, most commonly from bacteria or viruses that inflame the tiny air sacs in your lungs. Less often, fungi or inhaled chemicals are responsible. In the United States, pneumonia drives roughly 1.2 million emergency department visits each year and accounts for over 41,000 deaths annually. Understanding the different causes helps explain why some cases are mild and manageable at home while others become life-threatening.
How Pneumonia Develops in Your Lungs
Your airways are constantly exposed to germs, but a healthy immune system usually stops them before they reach deep into the lungs. Pneumonia takes hold when a pathogen overwhelms those defenses and reaches the alveoli, the grape-like clusters of air sacs where oxygen enters your blood. Once bacteria or viruses settle there, your body launches an inflammatory response. White blood cells rush to the site within three to four hours, and the surrounding blood vessels become leaky, allowing fluid and immune cells to flood the infected area.
That fluid buildup is what makes pneumonia dangerous. The air sacs that should be filled with air become filled with pus and inflammatory fluid instead, making it harder to breathe and reducing the oxygen that reaches your bloodstream. A mild case might feel like a bad chest cold, while a severe one can starve your organs of oxygen.
Bacterial Causes
Bacteria are the most common cause of pneumonia that requires medical treatment. The two leading culprits in community-acquired pneumonia (the kind you pick up in daily life, not in a hospital) are the bacterium behind pneumococcal disease and a smaller organism called Mycoplasma pneumoniae. Pneumococcal pneumonia tends to hit harder, with high fever, shaking chills, and rust-colored mucus. Mycoplasma infections are sometimes called “walking pneumonia” because they’re usually milder, causing a dry cough and fatigue that can drag on for weeks.
Several less common bacteria also cause pneumonia in adults. Legionella, the organism behind Legionnaires’ disease, spreads through contaminated water systems like cooling towers and hot tubs rather than person-to-person contact. Haemophilus influenzae and Chlamydia pneumoniae round out the list, along with pertussis (whooping cough), which can lead to pneumonia in adults whose childhood immunity has faded.
Viral Causes
Viruses cause a significant share of adult pneumonia cases, and the list extends well beyond COVID-19. Influenza remains one of the most frequent viral triggers, particularly during winter months. Respiratory syncytial virus (RSV), long known as a childhood illness, is now recognized as a serious pneumonia risk for adults over 60. Other respiratory viruses, including rhinoviruses and parainfluenza viruses, can also progress to pneumonia, especially in people with weakened immune systems or chronic lung disease.
Viral pneumonia often starts as a typical upper respiratory infection, with congestion, body aches, and a cough that gradually worsens rather than improving. One important complication: a viral infection can damage your airway lining enough to let bacteria move in, causing a secondary bacterial pneumonia on top of the original viral illness. This “double hit” is a major reason why flu-related pneumonia can turn serious quickly.
Fungal Causes
Fungal pneumonia is uncommon in healthy adults but poses a real threat to people with compromised immune systems. The specific fungal pathogen often depends on the type of immune deficiency involved. People with AIDS are particularly vulnerable to Pneumocystis jirovecii, a fungus that causes bilateral lung inflammation and is one of the defining infections of advanced HIV disease. Those with very low white blood cell counts (from chemotherapy, for example) are more susceptible to Aspergillus, a mold found in soil, decaying vegetation, and even hospital air systems.
Other fungal pathogens like Cryptococcus and Mucor species tend to affect people whose immune systems are suppressed by medications after organ transplants or by conditions that impair the body’s T-cells. Geographic fungi also matter: Histoplasma is common in the Ohio and Mississippi River valleys, while Coccidioides thrives in the desert Southwest. Healthy people can develop fungal pneumonia after heavy exposure to contaminated soil or bird droppings, though their cases are usually self-limiting.
Aspiration Pneumonia
Aspiration pneumonia develops when food, liquid, saliva, or stomach acid is accidentally inhaled into the lungs instead of swallowed into the stomach. The foreign material introduces bacteria and triggers inflammation. Most people occasionally inhale a tiny amount of saliva during sleep without consequences, but when larger volumes enter the lungs, or when the body can’t cough the material back out, infection follows.
Certain conditions make aspiration much more likely. Neurological problems like stroke, Parkinson’s disease, dementia, or brain injuries can impair the swallowing reflex or suppress the cough that would normally protect the airway. People who are heavily sedated from alcohol, drugs, or general anesthesia are also at elevated risk. Adults over 65 and residents of nursing homes or assisted living facilities develop aspiration pneumonia at higher rates, partly because of swallowing difficulties and partly because of other overlapping health issues.
Hospital-Acquired Pneumonia
Pneumonia that develops 48 hours or more after hospital admission is classified as hospital-acquired, and it’s caused by a different and often more dangerous set of bacteria than the community-acquired type. Patients on mechanical ventilators face the highest risk because the breathing tube bypasses the body’s natural airway defenses. The bacteria involved, including MRSA (methicillin-resistant Staphylococcus aureus), Pseudomonas aeruginosa, Acinetobacter, and certain strains of E. coli and Klebsiella, are frequently resistant to multiple antibiotics. That resistance makes hospital-acquired pneumonia harder to treat and significantly more dangerous, with higher mortality rates than pneumonia caught outside a hospital.
Chemical and Environmental Causes
Not all pneumonia is caused by an infection. Chemical pneumonitis is inflammation triggered by inhaling toxic fumes or irritants. Common culprits include chlorine gas from concentrated cleaning products, grain and fertilizer dust, pesticide fumes, and smoke from house fires or wildfires. Chronic aspiration of stomach acid, as can happen with severe gastroesophageal reflux, can also inflame lung tissue over time. Chemical pneumonitis doesn’t start as an infection, but the damaged lung tissue becomes vulnerable to bacterial invasion, so it can progress to infectious pneumonia if untreated.
Who Is Most at Risk
Age is the single strongest risk factor. Adults over 65 are far more likely to develop pneumonia and far more likely to die from it, both because the immune system weakens with age and because older adults tend to carry other conditions that compound the risk. Those conditions include COPD, asthma, bronchiectasis, heart failure, diabetes, liver or kidney disease, sickle cell disease, and malnutrition.
Anything that suppresses the immune system raises risk substantially. This includes HIV/AIDS, chemotherapy, long-term steroid use, and immunosuppressive drugs taken after organ or bone marrow transplants. Pregnancy also modestly suppresses immune function and increases susceptibility. Smoking damages the cilia that sweep pathogens out of your airways, making smokers significantly more vulnerable even if they have no other risk factors.
Vaccines That Reduce Your Risk
Vaccination is the most effective way to prevent the most dangerous forms of bacterial pneumonia. The CDC recommends pneumococcal vaccination for all adults 50 and older who have never received a pneumococcal conjugate vaccine. Several vaccine options exist: PCV20 and PCV21 are single-dose options that complete your pneumococcal vaccination series on their own. If PCV15 is used instead, a follow-up dose with a different pneumococcal vaccine is recommended about a year later. Adults under 50 with certain risk factors, including immune-suppressing conditions, cochlear implants, or cerebrospinal fluid leaks, are also recommended for earlier vaccination.
Beyond pneumococcal vaccines, staying current on your annual flu shot and COVID-19 boosters reduces the viral infections most likely to cause or lead to pneumonia. The newer RSV vaccines, approved for adults 60 and older, add another layer of protection. None of these vaccines eliminate pneumonia risk entirely, but they dramatically reduce the chances of severe illness and hospitalization.