Why is it that when u cough mucus comes out????10 points best answer!!!?!

mucus forms in your lungs and when you cough it will sometimes come out. it is a bacteria in your lungs. it is not really normal for mucus to come out though, so you might have some sort of infection; i have asthma so when i get a cold or even the slightest infection or even humidity makes mucus come out when i cough. especially when i go to the city. its nasty.