You get bronchitis when the airways in your lungs become inflamed, and the most common trigger is a viral infection. At least 90% of acute bronchitis cases are caused by viruses, the same ones responsible for colds and the flu. But infections aren’t the only route. Inhaled irritants, workplace exposures, and even acid reflux can inflame the bronchial tubes and produce that persistent, mucus-heavy cough.
Viral Infections Are the Primary Cause
The overwhelming majority of acute bronchitis starts with a virus. Rhinovirus (the common cold virus), influenza A and B, parainfluenza, coronavirus, respiratory syncytial virus, and enterovirus are the most frequently identified culprits. In most cases, what begins as a typical upper respiratory infection, a cold or flu, spreads down into the bronchial tubes and triggers inflammation there.
When a virus infects the lining of your airways, the tissue swells and your body ramps up mucus production. One type of mucus protein that normally stays at low levels increases significantly in response to irritants and infection, and this thicker, stickier mucus is harder for your airways to clear. That’s why a bronchitis cough can linger for weeks even after the initial infection resolves: the inflammation and excess mucus take time to settle down.
How Bronchitis Spreads Between People
Bronchitis itself isn’t contagious, but the viruses and bacteria behind it are. When someone with a respiratory infection coughs or sneezes, microscopic droplets carrying the pathogen launch into the air and can land in another person’s mouth, nose, or be inhaled into their lungs. The infection also spreads through surfaces. If a sick person touches a doorknob, railing, or shakes your hand, you can pick up the virus and introduce it to your body by touching your face.
You’re typically contagious until cold and flu symptoms like fever, sore throat, and active coughing have cleared. Viral bronchitis is more likely to spread than bacterial bronchitis.
Bacterial Bronchitis Is Less Common
Bacteria are detected in only 1% to 10% of acute bronchitis cases. The most common bacterial causes are organisms that also cause ear infections and pneumonia. A smaller percentage of cases involve atypical bacteria, including the one responsible for “walking pneumonia” and the bacterium that causes whooping cough. Whooping cough is worth knowing about because it can mimic a lingering bronchitis cough, and a booster vaccine exists for it.
Because bacterial bronchitis is relatively rare, antibiotics don’t help in most cases. This is one reason doctors often recommend rest and symptom management rather than prescribing medication for a typical bronchitis episode.
Smoking and Inhaled Irritants
Not all bronchitis comes from an infection. Long-term exposure to inhaled irritants is the main cause of chronic bronchitis, which is defined as a productive cough lasting at least three months in two consecutive years. Cigarette smoking is the leading cause, but secondhand smoke, air pollution, and chemical fumes or dusts from your home or workplace all contribute.
These irritants damage the airway lining over time, keeping it in a constant state of low-grade inflammation. The airways respond by overproducing mucus, which narrows the passages and makes breathing harder. Unlike acute bronchitis, which clears up, chronic bronchitis is a form of chronic obstructive pulmonary disease (COPD) and represents ongoing structural changes in the lungs.
Workplace Exposures That Raise Your Risk
Certain jobs put you at significantly higher risk. If you work in manufacturing, farming, animal care, food processing, cotton or textile production, mining, construction, or refining operations, you’re regularly breathing in dusts, fibers, and chemicals that irritate the bronchial tubes. Textile workers exposed to unprocessed cotton, hemp, or flax can develop a condition called brown lung disease. Workers in mines, foundries, and blasting operations inhale silica dust. Car garages expose mechanics to chemical fumes.
Organic dusts are another concern. Fungus spores, bacteria from moldy hay, bird droppings, and plant proteins can all trigger allergic lung inflammation. Agricultural workers and people in animal care face these exposures daily, and repeated irritation of the airways can lead to chronic bronchitis over time.
Acid Reflux as a Hidden Trigger
One lesser-known cause of recurring bronchitis is acid reflux, and it doesn’t have to be the obvious heartburn kind. Airway reflux involves a gaseous mist that rises from the stomach into the throat and airways. This mist can be partially or even entirely non-acid, which means you might not feel traditional reflux symptoms at all.
When this mist reaches the lower airways, it causes inflammation, excess mucus, tightening of the airways, and cough. The body’s response varies from person to person. Some develop a persistent cough with phlegm and wheezing that looks and feels identical to bronchitis caused by infection. In people with COPD, reflux-driven bronchitis can be a major contributor to ongoing symptoms. If you keep getting bronchitis without an obvious infection or irritant exposure, reflux is worth considering as a possible cause.
Who Is Most Vulnerable
Several factors make some people more likely to develop bronchitis than others. Smokers and people regularly exposed to secondhand smoke top the list. People with asthma or allergies already have sensitive, inflammation-prone airways, so a viral infection is more likely to settle into the bronchial tubes rather than staying in the nose and throat. Young children and older adults are more susceptible because their immune systems are either still developing or declining. A weakened immune system from any cause, whether illness, medication, or chronic stress, also lowers your defenses against the respiratory viruses that trigger most cases.
Seasonal timing matters too. Acute bronchitis peaks during fall and winter, when cold and flu viruses circulate most heavily and people spend more time indoors in close contact. Getting a flu vaccine each year reduces your chances of catching one of the viruses most commonly linked to bronchitis, and staying current on your pertussis booster helps protect against that bacterial cause as well.