A cough is a symptom of many diseases. Most coughs come from simple viral infections, such as the common cold. Sometimes, but not always, mucus is produced with the cough. If the colour is green or yellow, it may be a hint of a bacterial infection, although this is not always a reliable indicator. If the colour is red, there may be bleeding in the lungs. Any cough that produces blood or blood-stained mucus, as well as any cough that lasts more than two weeks, requires a visit to a medical professional for diagnosis.