Ear infections are most often caused by bacteria or viruses that get trapped in the middle ear after a cold or respiratory illness. About two-thirds of children under three have had at least one episode, making it one of the most common reasons for pediatric doctor visits. But ear infections aren’t just a childhood problem. Adults get them too, and the causes vary depending on which part of the ear is affected.
How a Cold Turns Into an Ear Infection
Most middle ear infections don’t start in the ear. They start with a viral upper respiratory infection, the common cold. Viruses like respiratory syncytial virus (RSV), rhinovirus, adenovirus, and parainfluenza damage the lining of the respiratory tract and trigger inflammation that swells the tissues around the Eustachian tube, a narrow channel connecting the middle ear to the back of the throat.
When that tube swells shut, it can no longer do its three main jobs: equalizing air pressure in the middle ear, draining fluid and debris toward the throat, and keeping bacteria from traveling up into the ear. Fluid builds up behind the eardrum with nowhere to go, creating a warm, stagnant environment where bacteria thrive. The result is pain, pressure, and sometimes fever as the immune system fights the growing infection.
The Bacteria Behind Most Cases
Three bacterial species cause more than 95% of middle ear infections. The most common is Streptococcus pneumoniae, followed by non-typeable Haemophilus influenzae and Moraxella catarrhalis. These bacteria commonly live in the nose and throat without causing problems, but when the Eustachian tube stops draining properly, they colonize the trapped fluid in the middle ear and multiply rapidly.
The viral infection that preceded the ear infection plays a double role here. Beyond blocking the Eustachian tube, it also weakens the local immune defenses by damaging the mucous membrane and ramping up inflammatory signals. This makes it easier for bacteria already present in the upper airway to establish a secondary infection.
Why Children Get Ear Infections So Often
The main reason is anatomy. A child’s Eustachian tube is significantly shorter and more horizontal than an adult’s, which makes it easier for bacteria and fluid to reach the middle ear and harder for that fluid to drain out. In children under two, the tube averages about 21 millimeters long. By age 15, it grows to around 27 millimeters. That difference matters: a shorter, flatter tube is more easily blocked by even mild swelling from a cold.
Children’s immune systems are also still learning to recognize and fight off common respiratory pathogens. They encounter new viruses constantly, especially in daycare settings, which means more colds and more opportunities for secondary ear infections. Roughly one-third of children who get an ear infection will have recurring episodes, defined as more than three in six months or more than four in a year.
Outer Ear Infections Have Different Causes
Not all ear infections involve the middle ear. Outer ear infections, sometimes called swimmer’s ear, affect the ear canal itself. In North America, 98% of these cases are bacterial, with Pseudomonas aeruginosa and Staphylococcus aureus being the two most common culprits. About one-third of cases involve multiple bacterial species at once.
The trigger is usually moisture or physical irritation that strips away the ear canal’s natural defenses. Earwax (cerumen) creates a slightly acidic, water-repellent barrier that keeps bacteria in check. When water sits in the canal after swimming or bathing, or when you scrape the skin with cotton swabs, earbuds, or hearing aids, that barrier breaks down. The pH of the canal rises, the skin softens and cracks, and bacteria move in. This is why outer ear infections spike during summer and in people who spend a lot of time in water.
Allergies and Acid Reflux
Allergic rhinitis (hay fever) is an underappreciated contributor to ear problems. Nasal allergies cause chronic inflammation and swelling of the mucous membranes, including those around the Eustachian tube opening. One clinical study found that Eustachian tube dysfunction was the most prevalent middle ear problem in people with allergic rhinitis, affecting 22% of patients evaluated. That dysfunction can lead to fluid buildup in the middle ear even without a bacterial infection, a condition sometimes called “glue ear.”
Gastroesophageal reflux disease (GERD) can cause similar problems. Stomach acid that travels up to the back of the throat irritates and inflames the tissue around the Eustachian tube, contributing to blockage. This is one reason some people with chronic reflux also report ear fullness or repeated ear infections.
Environmental Risk Factors
Secondhand smoke exposure meaningfully increases a child’s risk of middle ear infections. A meta-analysis of available studies found that children living with a mother who smokes had 62% higher odds of developing middle ear disease compared to unexposed children. Living with any household smoker raised the odds by 37%. The effect was even more pronounced when looking at children who eventually needed surgery for ear-related problems: maternal smoking nearly doubled those odds.
Other environmental factors that increase risk include:
- Daycare attendance, which exposes children to more respiratory viruses
- Bottle-feeding while lying flat, which can allow milk to flow toward the Eustachian tube
- Pacifier use, which is associated with slightly higher infection rates in some studies
- Fall and winter seasons, when respiratory viruses circulate more widely
Why Some Ear Infections Keep Coming Back
Recurring ear infections aren’t always caused by new bacteria entering the middle ear. Research published in JAMA found direct evidence of bacterial biofilms on the middle ear lining of children with chronic or recurrent infections. A biofilm is a colony of bacteria encased in a protective, self-produced matrix that adheres to tissue surfaces. This shield makes the bacteria far more resistant to antibiotics than free-floating bacteria would be.
Biofilms help explain a frustrating pattern many parents recognize: the ear infection seems to clear up with antibiotics, but comes back weeks later. The antibiotics may kill the exposed bacteria in the fluid but fail to penetrate the biofilm clinging to the middle ear lining. Once antibiotic treatment stops, the surviving bacteria re-emerge and trigger another episode.
The Role of Vaccines
Pneumococcal conjugate vaccines, given routinely to infants, were designed primarily to prevent serious pneumococcal diseases like meningitis and pneumonia. They also provide a modest reduction in ear infections. A Cochrane review found that the standard vaccine reduced all-cause middle ear infections by about 6% in healthy infants. An experimental 11-component version showed a larger 34% reduction, though it isn’t widely available. These numbers may sound small, but across millions of children, even a 6% reduction translates to a significant number of prevented infections and avoided antibiotic courses.
The limited effect makes sense when you consider that the vaccine targets only one of the three major bacterial causes. Ear infections triggered by Haemophilus influenzae or Moraxella catarrhalis, or those that follow purely viral illness, aren’t affected by the pneumococcal vaccine.