As I understand it, in a normal flu the body’s immune system attacks the replicating flu virus in the cells of the lungs by inflaming and sloughing off infected tissue full of that replicating virus, creating what we call mucous. But an otherwise healthy body can usually cough up and expel that mucous fast enough to keep ahead of it. With a nasty virus like COVID 19, its super-rapid replication rate overwhelms the lungs with so much mucous so fast that the coughing mechanism can’t cope, and the victim gets weak because the rest of their body including their brain and CNS are inflamed too, and the mucous gets backed up and the lungs fill up and – voila – pneumonia.