Ear infections in adults are most often caused by bacteria or viruses that get trapped in the ear when the body’s natural drainage and defense systems break down. Unlike childhood ear infections, which are extremely common, adult ear infections typically stem from a specific trigger: a cold, allergies, sinus problems, trapped moisture, or even acid reflux. The type of infection depends on where in the ear it develops, and each location has its own set of causes.
Middle Ear Infections and Blocked Drainage
The most familiar type of ear infection, called otitis media, happens in the air-filled space behind the eardrum. A narrow passage called the eustachian tube connects this space to the back of your throat, and its job is to equalize pressure and drain fluid. When the tube swells shut or gets clogged, fluid builds up in the middle ear, creating a warm, stagnant environment where bacteria thrive.
The two bacteria most commonly responsible are Streptococcus pneumoniae and nontypeable Haemophilus influenzae. Cold viruses are also a frequent cause. In most cases, the infection starts as a viral upper respiratory illness that inflames the eustachian tube, and bacteria move in once drainage stalls. Globally, middle ear infections account for roughly 391 million episodes per year across all age groups.
What Blocks the Eustachian Tube
Since eustachian tube blockage is the central mechanism behind middle ear infections, anything that causes swelling or congestion in this area raises your risk. The most common culprits include:
- Colds and flu: Viral infections inflame the lining of the tube, narrowing or closing it entirely.
- Allergies: Seasonal or year-round allergic rhinitis causes chronic swelling in the nasal passages and eustachian tube.
- Sinus infections: The inflammation and mucus buildup from sinusitis can block the tube directly. A prolonged blockage significantly increases the chance of a secondary ear infection.
- Acid reflux (GERD): The upper digestive tract and middle ear share a connection point in the upper throat called the nasopharynx. Stomach acid reaching this area can irritate and inflame the eustachian tube opening.
These conditions often overlap. Chronic sinusitis and GERD, for example, can feed into each other and both contribute to persistent eustachian tube dysfunction, which is why some adults get recurring ear infections that seem hard to pin down.
Outer Ear Infections
Outer ear infections, commonly called swimmer’s ear, develop in the ear canal rather than behind the eardrum. The ear canal has a built-in defense system: a thin film that repels water and limits bacterial growth. When that barrier breaks down, bacteria colonize the damp skin of the canal.
The two things that set the stage for outer ear infections are moisture and physical damage. Water left in the canal after swimming, heavy sweating, or prolonged humid weather creates an ideal breeding ground for bacteria. At the same time, cleaning your ears with cotton swabs, hairpins, or fingernails can scratch the canal’s lining and give bacteria a direct entry point. Earbuds and hearing aids can cause tiny skin breaks that do the same thing.
Outer ear infections are almost always bacterial, but fungi cause a meaningful minority of cases.
Fungal Ear Infections
Fungal ear infections, called otomycosis, are a distinct category. Aspergillus is responsible for about 90% of cases, with Candida causing the rest. These infections favor warm, humid conditions and peak during summer months in the United States.
You’re more likely to develop a fungal ear infection if you swim, surf, SCUBA dive, or spend a lot of time in water. People with weakened immune systems, ear eczema, or other skin conditions affecting the ear are also at higher risk. One less obvious factor: anything that reduces your earwax makes you more vulnerable. Earwax is mildly antifungal, so aggressively cleaning it out with cotton swabs or other tools removes a layer of natural protection.
Smoking and Immune Suppression
Cigarette smoke has a direct physical effect on the eustachian tube. The irritants in smoke can obstruct the tube and trap mucus and bacteria in the middle ear, essentially creating the same blocked-drainage scenario that colds and allergies produce. On top of that, long-term smoking weakens the immune system, making it harder for your body to fight off infections once they start. Secondhand smoke exposure carries similar risks.
Other forms of immune suppression, whether from chronic illness, medications, or conditions like diabetes, also increase susceptibility to both bacterial and fungal ear infections.
Pressure Changes and Barotrauma
Rapid changes in air pressure can stress the eustachian tube beyond its ability to adapt, leading to a condition called barotrauma. During airplane takeoff and descent, the cabin pressure shifts faster than the eustachian tube can equalize. This pressure mismatch prevents the eardrum from moving normally, causing pain, fluid buildup, and sometimes a secondary infection.
SCUBA diving produces even more dramatic pressure shifts and is a well-known trigger. Hyperbaric oxygen chambers can cause the same effect. If you already have eustachian tube dysfunction from allergies or a cold, you’re especially vulnerable to barotrauma during these activities, because the tube is already partially blocked and can’t compensate for the pressure change at all.
What Happens if an Ear Infection Goes Untreated
Most adult ear infections resolve with appropriate treatment, but infections that linger or go completely untreated can progress. The most concerning complication is mastoiditis, an infection of the bone behind the ear. Mastoiditis that isn’t adequately treated can lead to hearing loss, sepsis (a dangerous whole-body response to infection), meningitis, or brain abscess. These outcomes are rare, but they underscore why persistent ear pain, drainage, or hearing changes in adults shouldn’t be dismissed as a minor problem.
Repeated infections can also cause cumulative damage to the eardrum or the tiny bones of the middle ear, leading to gradual hearing loss that may not fully reverse even after the infections stop.