Doug That is a good question. I don't have the official or medical explanation. But I will tell you what I believe. It is a germ or even particles of something that get into your lungs. With all the airways, tubes, and air sacks it tends to get stuck in there and form the infection. The mucus or phlegm is your lungs way of clearing out all the particles and germs from our lungs. [Mostly happens at night while we're sleeping] That is why drinking a lot of water helps clear our lungs. When it docent get cleared out right away it becomes an infection and sticks to the lungs and we need antibiotics to clear it up.
Hope that helps you understand it a little better. @A MyCOPDTeam Member could probably explain it better for us.

@Gaillbsy COPD is a blanket term that covers any respiratory disease such as bronchitis, emphysema, pneumonia asthma and many others, so if you have been diagnosed with a respiratory disease that means that you have COPD; as @A MyCOPDTeam Member said, they are one and the same thing. Chris
August 18, 2024

Heres some explanations that I found on Pininterest….