Bronchitis occurs when the lining of your bronchial tract (the air tube that brings air to your lungs) becomes infected.
An infection occurs when microorganisms, usually from the air you breathe or develop from a cold or flu, cling to the lining of your bronchial tract and begin to multiply. As these bacteria multiply in numbers they begin to cause a significant irritation to the lining of your bronchial tract. These tubes become swollen, and begin producing mucus (phlegm) to fight the irritation this causes.
Unfortunately, this mucus also causes difficulty breathing, wheezing and a nasty cough. The infection itself will give you flu like symptoms.