Pneumonia in children is most often caused by viruses, especially in kids under age 5. Bacterial infections are the second most common cause, and in some cases, pneumonia develops when food, liquid, or stomach contents are accidentally inhaled into the lungs. The specific germ responsible often depends on a child’s age, and environmental factors like air pollution and secondhand smoke can make any child more vulnerable.
How Pneumonia Develops in the Lungs
Pneumonia is an infection of the air sacs deep in the lungs. When a virus or bacterium reaches those air sacs, the body’s immune response causes inflammation and fluid buildup. In the early phase, that fluid is thin and clear. Over the next one to two weeks, white blood cells and bacteria can accumulate, turning the fluid thicker and sometimes forming pus. This is what makes it hard for oxygen to pass from the lungs into the bloodstream, producing the cough, fast breathing, and fatigue that parents typically notice.
Children are particularly susceptible because their airways are smaller and their immune systems are still developing. A cold or upper respiratory infection that would stay in an adult’s nose and throat can travel deeper into a young child’s lungs and trigger pneumonia.
Viral Causes
Viruses account for the majority of pneumonia cases in young children. Respiratory syncytial virus (RSV) is the single most common viral cause. Other frequent culprits include influenza, parainfluenza, adenovirus, and human metapneumovirus. These viruses spread through respiratory droplets when an infected person coughs or sneezes, and through contact with contaminated surfaces.
Viral pneumonia tends to come on gradually, starting with cold-like symptoms that worsen over several days. It’s generally milder than bacterial pneumonia, though RSV can cause serious illness in infants and very young children. Most cases of viral pneumonia resolve on their own with supportive care.
Bacterial Causes
Streptococcus pneumoniae (pneumococcus) is the most common cause of bacterial pneumonia in children worldwide. Haemophilus influenzae type b (Hib) is the second most common. Bacterial pneumonia often produces higher fevers, more labored breathing, and a sicker-looking child compared to viral cases. It can also develop as a secondary infection after a viral illness weakens the lungs’ defenses.
In newborns, the bacterial picture looks different. Group B strep, which a baby can pick up during delivery, is one of the leading causes. Herpes simplex virus can also cause pneumonia in the newborn period, though this is less common.
Children with HIV face a unique risk. A fungal organism called Pneumocystis jiroveci is one of the most common causes of pneumonia in HIV-infected infants, responsible for at least one quarter of all pneumonia deaths in that group.
Walking Pneumonia in School-Age Kids
Parents of school-age children often hear the term “walking pneumonia.” This is a milder form of pneumonia caused by a bacterium called Mycoplasma pneumoniae. It most commonly affects school-aged children and young adults, and it spreads through coughing and sneezing in close-contact settings like classrooms and homes.
Walking pneumonia earns its name because kids usually aren’t sick enough to stay in bed. They may have a persistent dry cough, low-grade fever, and general fatigue, but they can often keep going to school. Brief, casual contact with an infected person doesn’t usually spread the infection. The real risk is prolonged close contact. During school outbreaks, family members of sick children are the most likely people in the community to get infected next.
Aspiration Pneumonia
Not all pneumonia comes from catching a germ. Aspiration pneumonia happens when food, saliva, liquid, or stomach contents are inhaled into the lungs instead of swallowed into the stomach. The inhaled material irritates the lung tissue and can introduce bacteria from the mouth or stomach.
This type of pneumonia is most common in children who have swallowing difficulties, a condition called dysphagia. Several medical situations can lead to dysphagia in kids:
- Structural differences like cleft palate or esophageal abnormalities
- Neurological conditions such as cerebral palsy or brain injury
- Developmental delays from premature birth or conditions like Down syndrome
- Neuromuscular diseases like spinal muscular atrophy
- Gastroesophageal reflux (GERD), where stomach contents flow back up into the throat
Children who have a nasogastric tube or tracheostomy are also at higher risk for aspiration.
Environmental Risk Factors
The germs that cause pneumonia are only part of the equation. A child’s environment plays a major role in whether exposure to those germs actually leads to infection.
Air pollution is one of the most significant risk factors globally. After childhood malnutrition, household and outdoor air pollution are the second and third highest contributors to pneumonia deaths in young children. In 2021, an estimated 709,000 deaths in children under 5 were linked to air pollution. Roughly 72% of those were tied to household air pollution from solid cooking fuels (wood, charcoal, dung), while 28% were linked to outdoor particulate matter. Exposure to household air pollution nearly doubles a child’s risk of lower respiratory tract infection.
The biological mechanism is straightforward. Smoke and particulate matter slow down the tiny hair-like structures in the airways that sweep mucus and germs out of the lungs. At the same time, pollution increases mucus production, damages the lining of the airways, and impairs the immune cells responsible for killing bacteria. Research has shown that particulate matter actually helps pneumococcal bacteria stick to and invade airway cells more effectively. In other words, polluted air doesn’t just weaken a child’s defenses; it gives the bacteria an advantage.
Secondhand tobacco smoke works through similar pathways. Children exposed to postnatal tobacco smoke carry higher levels of pneumococcal bacteria in their noses and throats, which increases their chances of developing pneumonia. Other environmental and social risk factors include childcare attendance, living with other young children, poverty, and seasonal patterns (pneumonia peaks in colder months in temperate climates).
How Vaccines Have Changed the Picture
Vaccination has dramatically reduced bacterial pneumonia in children. Pneumococcal conjugate vaccines target the most common strains of Streptococcus pneumoniae, and the Hib vaccine protects against Haemophilus influenzae type b. Both are part of the routine childhood immunization schedule in most countries.
Newer pneumococcal vaccines cover more bacterial strains than earlier versions. Modeling research estimates that switching from a 15-strain vaccine to a 20-strain vaccine could prevent an additional 889,000 cases of pneumococcal pneumonia and 3.1 million cases of ear infections over a 10-year period, with billions of dollars in healthcare savings. These vaccines don’t prevent viral pneumonia, but by reducing the bacterial infections that often complicate viral illness, they lower the overall burden significantly.