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What causes most ear infections?

Ear infections are one of the most common conditions affecting children. They occur when fluid builds up in the middle ear, causing pain, fever, and temporary hearing loss. Understanding the causes of ear infections can help parents prevent and properly treat them.

The Eustachian Tube

To understand what causes most ear infections, it helps to first understand how the ear is structured. The ear canal leads to the eardrum, which separates the outer ear from the middle ear. The middle ear is connected to the back of the throat by a narrow passageway called the Eustachian tube. The Eustachian tube helps regulate air pressure in the middle ear and drain fluid out of the ears into the throat.

When the Eustachian tube becomes blocked or impaired, either from swelling due to allergies or infection, fluid can build up in the middle ear space. Bacteria or viruses trapped in this fluid can multiply, causing an ear infection. The most common causes of Eustachian tube blockage in children are:

  • Allergies
  • Enlarged adenoids
  • Colds and other respiratory infections
  • Exposure to cigarette smoke

Bacterial vs Viral Infections

Ear infections can be caused by both bacteria and viruses. Bacterial infections are more likely to cause pain, fever, and inflammation, while viral infections are more likely to cause congestion and upper respiratory symptoms. Here are some of the most common bacterial and viral culprits:

Bacterial Infections

  • Streptococcus pneumoniae – causes about 50% of bacterial ear infections
  • Haemophilus influenzae
  • Moraxella catarrhalis

Viral Infections

  • Rhinovirus – common cold virus
  • Influenza
  • Respiratory syncytial virus (RSV)
  • Parainfluenza
  • Adenovirus
  • Enterovirus


Age is one of the biggest risk factors for ear infections. Children under age 3, especially between 6 months and 2 years, are most susceptible. The reasons are:

  • Their Eustachian tubes are shorter and straighter than adult tubes, allowing bacteria and viruses to reach the middle ear more easily.
  • Their immune systems are still developing, making them more vulnerable to infections.

As children grow and their Eustachian tubes lengthen and angle downwards, their risk of infections decreases. By age 10, most outgrow their susceptibility to ear infections.

Other Risk Factors

While age is the primary risk factor, other things that can increase a child’s risk of developing an ear infection include:

  • Being in daycare – increased exposure to bacteria and viruses
  • Pacifier use – can increase fluid buildup in the ear
  • Bottle feeding while lying down – allows formula or milk to flow into the Eustachian tubes
  • Genetic predisposition or anatomy that obstruct Eustachian tubes
  • Exposure to secondhand smoke
  • Allergies and asthma
  • Immune deficiency
  • Cleft palate or Down syndrome
  • Previous ear infection – increases risk of recurrence


Ear infections tend to be seasonal, peaking in the fall and winter when colds and upper respiratory infections are more common. The winter months see around 30% more cases of ear infections compared to the summer. The back-to-school season in the fall is another peak time when kids are exposed to more germs and viruses in the classroom. The seasonal nature highlights the key role colds and respiratory illnesses play in many ear infections.


Diagnosing the cause of an ear infection involves a physical exam and questions about symptoms. Doctors use an otoscope to look inside the ears for fluid buildup, redness, and signs of infection. They may also check for fever and swollen lymph nodes around the neck. A tear in the eardrum from intense pressure may also be visible.

To determine the exact bacterium or virus responsible, a sample of fluid from inside the ear would need to be collected and analyzed. This is not routine and only done in severe, chronic, or uncertain cases.

Sometimes a hearing test is conducted after an ear infection to ensure the infection did not cause lasting hearing damage. Such tests are recommended for children who have speech or developmental delays.

Bacterial vs Viral Treatment

While the precise bacterium or virus cannot always be identified, doctors can usually determine if the infection is bacterial or viral based on examination and symptoms. This guides treatment:

Bacterial infections

Are treated with antibiotics, usually amoxicillin as a first choice. Stronger antibiotics may be prescribed if symptoms worsen or persist.

Viral infections

Do not respond to antibiotics. Treatment focuses on managing pain and fever with over-the-counter medications. Decongestants and antihistamines may be recommended for stuffiness. Viral infections tend to resolve on their own as the immune system fights off the virus.


While some ear infections are inevitable, there are things parents can do to lower the risks:

  • Get recommended vaccines – pneumococcal and influenza vaccines protect against bacteria and viruses that cause ear infections.
  • Wash hands frequently and disinfect surfaces – reduces spread of bacteria and viruses.
  • Avoid smoking around children – minimizes smoke irritation of the respiratory tract.
  • Don’t allow sick children around infants – keeps them away from contagious viruses.
  • Breastfeed infants for at least 6 months -boosts their immunity.
  • Wean bottle-fed infants off bottles by 12 months – prevents milk from flowing into ears.
  • Don’t allow children to sleep with feeding bottles – keeps ears clear of fluids.

While these steps may not always prevent infection, they can reduce the frequency and severity of ear infections.


Ear infections most often arise when bacteria or viruses infiltrate the middle ear, especially via a blocked Eustachian tube. Children under 3 are most susceptible due to their developing anatomy and immunity. Respiratory illnesses like colds frequently precede or coincide with ear infections. While antibiotics treat bacterial causes, viral infections must run their course. Preventive strategies focus on hygiene, vaccination, avoiding smoke exposure, and eliminating prolonged bottle feeding during sleep.