Ear infections in adults are most often caused by bacteria or viruses that get trapped in the middle or outer ear, usually after a cold, allergies, or water exposure. While far less common than in children (five out of six kids get at least one by age three), adult ear infections can be more painful and slower to resolve because the underlying causes tend to be chronic or recurring.
There are three main types of ear infections, each with different triggers: middle ear infections, outer ear infections, and fungal ear infections. Understanding which part of the ear is affected helps explain why the infection started and what keeps it coming back.
Middle Ear Infections and Eustachian Tube Problems
The most common setup for a middle ear infection is a blocked or swollen eustachian tube. This narrow channel connects your middle ear to the back of your throat and is responsible for draining fluid and equalizing pressure. When it swells shut, fluid pools behind the eardrum and creates a warm, stagnant environment where bacteria multiply quickly.
Several things cause eustachian tube swelling in adults:
- Upper respiratory infections: Colds and flu are the most frequent trigger, spreading inflammation from the throat and nasal passages to the tube.
- Allergies: Seasonal or year-round allergic rhinitis keeps the tissue around the tube chronically inflamed.
- Acid reflux (GERD): Stomach acid reaching the back of the throat can irritate and swell the tissue near the tube opening.
- Altitude changes: Flying, scuba diving, or driving through mountains creates pressure differences that stress the eustachian tube, a condition called barotrauma.
The bacteria responsible for adult middle ear infections look a lot like the ones found in children. A study of middle ear fluid from 34 adults found that nontypeable Haemophilus influenzae caused 26% of cases, Streptococcus pneumoniae caused 21%, and smaller shares were attributed to Moraxella catarrhalis, Group A streptococcus, and Staphylococcus aureus. COVID-19 has also been identified as a cause, with SARS-CoV-2 detected in middle ear fluid in several cases since the pandemic began.
One notable finding: Group A streptococcus is an uncommon cause of middle ear infections overall, but when it does occur in adults, it tends to be severe. One study found it was the single most common pathogen in adults hospitalized for ear infections, responsible for 15% of those serious cases.
Outer Ear Infections (Swimmer’s Ear)
Outer ear infections affect the ear canal, the passage between your outer ear and eardrum. The ear canal is the only skin-lined dead-end passage in the human body, which means it traps moisture easily and creates an ideal environment for bacteria and fungi to grow.
Water is the most obvious trigger. When water gets trapped in the ear canal after swimming, showering, or surfing, it softens the skin lining and disrupts the canal’s natural acidity. Bacteria that normally live harmlessly on the skin surface can then penetrate the softened tissue and cause infection.
But you don’t need to swim to get an outer ear infection. The other major cause is mechanical damage to the ear canal. Cotton swabs are a frequent culprit. Inserting them scrapes away the thin layer of protective earwax and can create tiny breaks in the skin, giving bacteria a direct entry point. Earbuds, fingernails, and other objects do the same thing.
Skin conditions also play a role. Eczema and other forms of dermatitis can affect the ear canal, causing dry, cracked, or inflamed skin that’s vulnerable to infection. If you have a skin condition that flares inside your ears, recurring outer ear infections are a predictable consequence.
Hearing Aids, Earbuds, and Device-Related Infections
Anything you wear inside your ear canal regularly changes the bacterial environment. Hearing aids are a well-documented risk factor for outer ear infections. The ear mold traps humidity against the canal skin, and a wide range of bacteria and fungi colonize the hearing aid surface over time. Research has shown that wearing hearing aids measurably alters the normal microbial balance in the ear canal, creating conditions that favor infection.
The same principle applies to earbuds, earplugs, and in-ear monitors. If you wear any of these for long stretches, especially in warm weather or during exercise, you’re sealing moisture into a space that was designed to stay dry and ventilated. Cleaning your devices regularly and giving your ears breaks reduces this risk.
Fungal Ear Infections
About 10% of outer ear infections are caused by fungi rather than bacteria. Aspergillus is responsible for roughly 90% of fungal ear infections, with Candida causing most of the rest. These infections thrive in warm, humid conditions and are most common during summer months and in people who spend time in water.
Fungal infections look and feel different from bacterial ones. An Aspergillus infection often produces yellow or black dots with fuzzy white patches visible in the ear canal. A Candida infection tends to cause a thick, creamy white discharge. Both types cause intense itching, which often distinguishes them from the sharper pain of bacterial infections.
Ironically, antibiotic ear drops used for bacterial infections can set the stage for a fungal infection by wiping out competing bacteria and leaving fungi free to spread. People with weakened immune systems or diabetes are also at higher risk.
Diabetes and Severe Infections
Diabetes deserves special attention because it doesn’t just increase the risk of ear infections; it increases the risk of dangerous ones. A condition called necrotizing otitis externa (sometimes called malignant otitis externa) occurs when an outer ear infection spreads from the skin into the bone and cartilage of the skull base. This is rare in the general population but disproportionately affects people with diabetes.
In one population-based study from Taiwan, 54.8% of patients with necrotizing otitis externa had a prior diabetes diagnosis. A larger study of 355 patients found that 74.6% had diabetes. The connection is straightforward: diabetes impairs blood flow to small vessels and weakens the immune response, allowing bacteria (often Pseudomonas) to invade tissue that would normally contain them. In a study of 88 patients, every patient who died from this condition had diabetes.
Chronic and Recurring Infections
Some adults get ear infections repeatedly, and the cause is often structural rather than a single exposure to bacteria. A cholesteatoma, an abnormal growth of skin cells behind the eardrum, is one common reason. Cholesteatomas can develop after eardrum damage from injury, previous infections, or ear surgery. They create a pocket that traps dead skin and debris, providing a persistent breeding ground for bacteria. If you notice foul-smelling drainage from one ear along with gradual hearing loss, a cholesteatoma is a possibility worth investigating.
Chronic eustachian tube dysfunction is another driver of recurrent infections. Some adults have tubes that are naturally narrow or that stay inflamed due to ongoing allergies or sinus problems. Each time the tube fails to drain properly, fluid accumulates, and the cycle repeats. Treating the underlying allergy or reflux is often more effective than treating the infections one at a time.
Smoking is also linked to recurring ear problems. Tobacco smoke irritates the mucous membranes of the upper airway and impairs the tiny hair-like cells that sweep mucus and pathogens out of the eustachian tube, making infections more likely to develop and harder to clear.