Acute bronchitis is an infection of the bronchial (say: “brawn-kee-ull”) tree. The bronchial tree is made up of series of small tubes that bring air into your lungs. When these tubes get infected, they swell up and form a thick fluid called mucus on the inside them. This makes it hard for you to breathe. You may cough up mucus and wheeze when you try to breathe. Wheezing is when make a whistling sound when you inhale or exhale.