Acute and chronic bronchitis is typically caused by the same virus as colds and flu, bacteria, pollution, and smoking, in both your body is fighting germs but your make more mucus causing your bronchial tubes to swell which makes it harder to breathe. You are at a higher risk if you smoke or breathe in second-hand smoke for a long time. If you have a weaker immune system, it can put you at a higher risk for bronchitis.