Pneumonia develops when bacteria, viruses, or fungi infect the lungs and cause the tiny air sacs to fill with fluid and pus. You can get it by breathing in infected droplets, aspirating fluid from your mouth or stomach into your lungs, or catching a respiratory virus that spreads deeper than usual. In the United States, pneumonia sends about 1.2 million people to the emergency department each year and causes over 41,000 deaths annually.
How Germs Reach Your Lungs
There are two main routes. The first is inhaling airborne droplets that contain bacteria or viruses, the same way you catch a cold or the flu. Someone nearby coughs or sneezes, you breathe in those particles, and if the germ reaches deep enough into your airways, it can settle in the air sacs of your lungs and start multiplying.
The second route is actually more common for bacterial pneumonia: aspiration of fluid already in your mouth or throat. Your mouth naturally contains bacteria, and tiny amounts of saliva slip past your vocal cords into your lungs more often than you’d think, especially during sleep. A healthy body clears these small exposures easily. But when a larger volume gets through, or when your immune defenses are weakened, those bacteria can overwhelm your lungs and trigger an infection.
In rarer cases, a bloodstream infection elsewhere in the body can carry pathogens to the lungs.
What Happens Inside Your Lungs
When a pathogen reaches the air sacs (alveoli), immune cells called macrophages try to destroy it. In the process, they release chemical signals that recruit waves of additional immune cells to the area. This immune response is what actually causes pneumonia’s damage. The flood of immune activity makes the tiny blood vessels in the lungs “leaky,” allowing fluid to seep into the air sacs. That fluid buildup is why breathing becomes difficult and painful. The air sacs that normally exchange oxygen for carbon dioxide are now partially or fully filled with inflammatory fluid and debris.
Bacteria, Viruses, and Fungi
The germ behind your pneumonia matters because it determines how the infection behaves and how it’s treated.
Bacteria are the most common cause of pneumonia requiring hospitalization. Streptococcus pneumoniae (pneumococcus) is the leading bacterial culprit in the United States. Mycoplasma pneumoniae causes a milder form often called “walking pneumonia,” where you feel sick but can usually stay on your feet. Legionella causes a severe form called Legionnaires’ disease, typically contracted from contaminated water systems rather than from other people.
Viruses cause a large share of pneumonia cases, especially in children. Influenza and the common cold virus (rhinovirus) are the most frequent viral causes in adults. RSV is the top viral cause in young children. SARS-CoV-2, the virus behind COVID-19, can also cause pneumonia, sometimes with unusually widespread lung involvement.
Fungi are a less common cause, mainly a threat to people with weakened immune systems. Pneumocystis jirovecii is one of the most recognized fungal causes and primarily affects people with HIV or those on immune-suppressing medications.
Aspiration Pneumonia
Aspiration pneumonia deserves its own explanation because it doesn’t come from “catching” something. It happens when you inhale bacteria-rich fluid from your throat or stomach contents into your lungs. This can occur during vomiting, a seizure, heavy sedation, or any situation where normal swallowing reflexes are impaired.
People at highest risk include those with neurological conditions like Parkinson’s disease, dementia, or the aftermath of a stroke, all of which can disrupt the precise coordination needed to swallow safely. Heavy alcohol use, drug overdoses, and sedative medications also suppress the reflexes that normally keep fluid out of the airways. Older adults face additional risk because aging reduces throat sensation, slows the cough reflex, and can weaken the muscles that close off the airway during swallowing.
Gastroesophageal reflux, esophageal disorders, and being bedridden for extended periods also increase the chances of aspiration.
Community vs. Hospital Settings
Where you pick up pneumonia changes what you’re likely dealing with. Community-acquired pneumonia, the kind you develop in everyday life, is most often caused by pneumococcus, Haemophilus influenzae, or Legionella. These bacteria generally respond well to standard antibiotics.
Hospital-acquired pneumonia is a different challenge. People who develop pneumonia during or shortly after a hospital stay are more likely to be infected with antibiotic-resistant organisms like MRSA, Pseudomonas, or Acinetobacter. These infections are harder to treat and tend to be more severe, partly because hospitalized patients are already in a weakened state.
Who Is Most Vulnerable
Age is one of the strongest risk factors. Children under 5 and adults over 65 are significantly more likely to develop pneumonia and to have serious complications from it. Young children’s immune systems are still maturing, while older adults experience a gradual decline in immune function and lung defenses.
Chronic conditions raise your risk substantially. Lung diseases like COPD or asthma compromise the airways’ ability to clear pathogens. Heart disease, diabetes, and kidney disease weaken the body’s overall immune response. Anything that suppresses the immune system, whether it’s chemotherapy, organ transplant medications, or HIV, makes it easier for organisms that a healthy immune system would handle to gain a foothold in the lungs.
Smoking damages the cilia, the tiny hair-like structures that sweep debris and mucus out of your airways. Without that cleaning mechanism working properly, bacteria and viruses have a much easier path into the deeper lung tissue.
Environmental Exposures
Air pollution is a recognized contributor to pneumonia risk. The World Health Organization lists pneumonia among the diseases most strongly linked to air pollution exposure. Fine particulate matter, particles small enough to travel deep into the lungs and even enter the bloodstream, is especially harmful. These particles trigger inflammation, suppress local immune defenses, and damage lung tissue, all of which make it easier for an infection to take hold.
Household air pollution from cooking with solid fuels like wood or charcoal poses a significant risk in many parts of the world, particularly for children and women who spend hours near the smoke. Even desert dust storms can spike particulate matter levels enough to increase respiratory infections in affected regions.
How Quickly Pneumonia Develops
The timeline from exposure to symptoms varies by the type of germ. Bacterial pneumonia caused by pneumococcus has a short incubation period of just 1 to 3 days, and symptoms tend to hit suddenly: high fever, shaking chills, and chest pain that worsens with breathing. Viral pneumonia often develops more gradually over several days, sometimes starting as a typical upper respiratory infection that slowly worsens and moves deeper into the chest. Walking pneumonia from Mycoplasma can come on slowly too, with a dry cough, low-grade fever, and fatigue that builds over a week or more.
Reducing Your Risk
Vaccination is the most effective preventive measure for bacterial pneumonia. The CDC recommends pneumococcal vaccines for all children under 5 (a four-dose series starting at 2 months old) and for all adults 50 and older. For adults, a single dose of PCV20 or PCV21 completes the series with no follow-up dose needed.
Annual flu vaccination also matters, since influenza is one of the top causes of viral pneumonia and a flu infection can pave the way for a secondary bacterial pneumonia on top of it. Staying current on COVID-19 vaccines reduces the risk of SARS-CoV-2 pneumonia.
Beyond vaccines, basic respiratory hygiene makes a real difference: frequent handwashing, avoiding close contact with visibly sick people, and not smoking. For people at risk of aspiration, keeping the head of the bed elevated, maintaining good oral hygiene to reduce bacteria in the mouth, and working with a speech therapist on safe swallowing techniques can meaningfully lower the chance of aspiration pneumonia.