Lung infections are caused by bacteria, viruses, fungi, or inhaled chemical irritants that trigger inflammation in the airways or lung tissue. In 2023, lower respiratory infections were responsible for an estimated 2.5 million deaths worldwide, with children under 5 and adults over 70 carrying the highest burden. Understanding the specific causes helps explain why some lung infections are mild and self-limiting while others become life-threatening.
How Pathogens Reach Your Lungs
Your lungs have built-in defenses: tiny hairs in your airways filter particles, mucus traps microbes, and coughing expels what doesn’t belong. Infections happen when those defenses are overwhelmed or bypassed. The three main entry routes are inhaling airborne droplets or spores from an infected person or contaminated environment, aspirating (accidentally breathing in) small amounts of saliva or stomach contents that carry bacteria, and spreading from an existing infection elsewhere in the body through the bloodstream.
Aspiration is more common than most people realize. It happens during sleep, after drinking alcohol, or in people with neurological conditions that affect swallowing. Altered mental status during intoxication, for instance, creates easy opportunities for mouth bacteria to slip into the lungs.
Bacterial Causes
Bacteria are the most deadly category of lung infection pathogens. Streptococcus pneumoniae (the pneumococcal bacterium) accounts for roughly 634,000 lower respiratory infection deaths per year globally, making it the single largest bacterial killer in this category. Staphylococcus aureus follows at about 271,000 deaths, and Klebsiella pneumoniae at about 228,000.
Community-acquired bacterial pneumonia, the kind you pick up in everyday life rather than in a hospital, is most commonly caused by Mycoplasma pneumoniae and Streptococcus pneumoniae. Less common bacterial causes include Chlamydia pneumoniae, Haemophilus influenzae, and Legionella (the bacterium behind Legionnaires’ disease, which spreads through contaminated water systems like cooling towers and hot tubs).
Atypical Bacterial Pneumonia
Some bacterial lung infections behave differently from the “classic” pneumonia most people picture. Atypical pneumonia tends to produce milder respiratory symptoms but also causes problems that seem unrelated to the lungs: headache, low-grade fever, rash, joint pain, diarrhea, and vomiting. Mycoplasma pneumonia, sometimes called “walking pneumonia,” often affects people under 40 and can cause ear pain, eye soreness, and sore throat alongside the cough. Chlamydia pneumonia occurs year-round and is generally mild.
Legionella pneumonia is the exception to the “milder” pattern. It tends to hit middle-aged and older adults, smokers, and people with chronic illnesses hardest. Symptoms typically worsen over the first four to six days before improving over another four to five days, and patients may cough up bloody mucus.
Viral Causes
Viruses that start as upper respiratory infections (affecting the nose and throat) can progress deeper into the lungs and cause pneumonia or bronchitis. The most common culprits are influenza (the flu), COVID-19, and RSV (respiratory syncytial virus). Adenovirus, parainfluenza, and even rhinovirus (the common cold virus) can also cause lung infections, particularly in young children, older adults, or people with weakened immune systems.
Viral lung infections often set the stage for a secondary bacterial infection. The virus damages the lining of the airways, making it easier for bacteria already present in your mouth and throat to colonize the lungs. This is why bacterial pneumonia frequently follows a bout of the flu.
Fungal Causes
Fungal lung infections were long considered a problem only for people with severely weakened immune systems, such as organ transplant recipients or those undergoing chemotherapy. That picture has changed. These infections are now increasingly recognized in people with healthy immune systems, particularly when there’s underlying lung damage, temporary immune disruption, or heavy exposure to fungal spores.
The main fungi involved include Aspergillus (a mold found in soil, decaying vegetation, and construction dust), Cryptococcus (often linked to bird droppings), and a group of “endemic” fungi found in specific geographic regions: Histoplasma in the Ohio and Mississippi River valleys, Blastomyces in the Great Lakes and Ohio River regions, and Coccidioides in the desert Southwest. If you live in or travel through these areas, breathing in disturbed soil or dust can introduce spores into your lungs.
People with structural lung problems like COPD or bronchiectasis are at particular risk for chronic fungal infections, especially aspergillosis, which can take several forms ranging from an allergic reaction to an invasive infection that destroys lung tissue.
Chemical and Irritant Causes
Not all lung inflammation comes from living organisms. Chemical pneumonitis is inflammation caused by inhaling toxic fumes or accidentally aspirating chemicals. Common household and workplace culprits include chlorine gas (from cleaning products, pool chemicals, or industrial exposure), 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, also causes chemical pneumonitis over time.
Chemical pneumonitis isn’t technically an “infection” since no pathogen is involved, but it damages the lung lining in ways that mimic infection and can quickly lead to a real one. Inflamed, injured lung tissue becomes a welcoming environment for bacteria.
Risk Factors That Increase Susceptibility
Certain conditions make your lungs more vulnerable to infection regardless of the specific pathogen involved:
- Smoking reduces your airways’ ability to clear mucus, removing one of the lungs’ primary defenses.
- Chronic lung diseases like asthma, COPD, cystic fibrosis, and bronchiectasis alter lung structure in ways that trap pathogens.
- Weakened immune systems from HIV/AIDS, organ transplants, chemotherapy, long-term steroid use, or pregnancy make it harder to fight off invaders.
- Neurological conditions such as stroke, dementia, Parkinson’s disease, or head injuries can impair your cough reflex or swallowing ability, increasing aspiration risk.
- Other chronic illnesses including diabetes, heart failure, sickle cell disease, malnutrition, and liver or kidney disease all raise pneumonia risk through various mechanisms.
- Hospitalization, especially time in an intensive care unit or on a ventilator, exposes you to aggressive hospital-acquired bacteria that are often resistant to common antibiotics.
Drug and alcohol use also weakens the immune system and impairs consciousness, creating a dual risk of reduced defenses and increased aspiration.
How Age Shapes Risk
Age is one of the strongest predictors of lung infection severity. Children under 5 have immune systems that are still developing, making them especially vulnerable to RSV, influenza, and bacterial pneumonia. The good news is that mortality in this age group has dropped by about 33% since 2010, largely due to improved vaccination and nutrition programs in lower-income countries.
Adults over 70 face the opposite problem: immune function naturally declines with age (a process called immunosenescence), and decades of accumulated lung exposure, chronic disease, and medication use compound the risk. This is why pneumococcal vaccination is now recommended for all adults 50 and older. Current vaccines come in several formulations that cover different strains of pneumococcal bacteria, and depending on which version you receive, you may need one dose or a two-dose series spaced about a year apart.
Reducing Your Risk
Vaccination is the most effective tool for preventing bacterial and viral lung infections. Pneumococcal vaccines are recommended for children under 5 and adults 50 and older. Annual flu vaccines and staying current on COVID-19 boosters reduce viral pneumonia risk. For children and high-risk adults, RSV vaccines or preventive treatments are also available.
Beyond vaccination, the basics matter: not smoking (or quitting), managing chronic conditions like diabetes and COPD, practicing good hand hygiene, and avoiding close contact with people who are actively sick. If you have a condition that affects swallowing, working with a speech therapist can reduce aspiration risk. And if your work exposes you to dust, fumes, or chemical irritants, proper respiratory protection is essential.