Picture of Bronchitis
Acute bronchitis usually comes on quickly and gets better after several weeks. This form of bronchitis is responsible for the hacking cough and phlegm/mucus production that sometimes accompany an upper respiratory infection. In most cases the infection is viral in origin, but sometimes it's caused by bacteria. If you are otherwise in good health, the mucous membrane will return to normal after you've recovered from the initial lung infection, which usually lasts for several days.
Image Source: MedicineNet
Text: "Understanding Bronchitis", MedicineNet