Bronchitis is an infection of the main airways of the lungs (bronchi), causing them to become irritated and inflamed.The main symptom is a cough, which may bring up yellow-grey mucus (phlegm). Bronchitis may also cause a sore throat and wheezing.Most cases of bronchitis can be treated easily at home with rest, non-steroidal anti-inflammatory drugs (NSAIDs) and plenty of fluids.You only need to see your GP if your symptoms are severe or unusual - for example, if: -your cough is severe or lasts longer than three weeks-you have a constant fever (a temperature of 38°C - 100.4°F - or above) for more than three days-you cough up mucus streaked with blood-you have an underlying heart or lung condition, such as asthma or heart failureYour GP may need to rule out other lung infections, such as pneumonia, which has symptoms similar to those of bronchitis. If your GP thinks you may have pneumonia, you will probably need a chest X-ray, and a sample of mucus may be taken for testing.If your GP thinks you might have an undiagnosed underlying condition, they may also suggest a pulmonary function test. You will be asked to take a deep breath and blow into a device called a spirometer, which measures the volume of air in your lungs. Decreased lung capacity can indicate an underlying health problem.In most cases, bronchitis will clear up by itself within a few weeks without the need for treatment. This type of bronchitis is known as "acute bronchitis". While you are waiting for it to pass, you should drink lots of fluid and get plenty of rest.In some cases, the symptoms of bronchitis can last much longer. If symptoms last for at least three months, it is known as "chronic bronchitis". There is no cure for chronic bronchitis, but there are several medications to help relieve symptoms. It is also important to avoid smoking and smoky environments, as this can make your symptoms worse.The bronchi are the main airways in your lungs, which branch off on either side of your windpipe (trachea). They lead to smaller and smaller airways inside your lungs, known as bronchioles.The walls of the bronchi produce mucus to trap dust and other particles that could otherwise cause irritation.Most cases of acute bronchitis develop when an infection causes the bronchi to become irritated and inflamed, which causes them to produce more mucus than usual. Your body tries to shift this extra mucus through coughing.Smoking is the most common cause of chronic bronchitis. Over time, tobacco smoke can cause permanent damage to the bronchi, causing them to become inflamed.Pneumonia is the most common complication of bronchitis. It happens when the infection spreads further into the lungs, causing air sacs inside the lungs to fill up with fluid. 1 in 20 cases of bronchitis lead to pneumonia.