COVID-19
The expanding part of the lungs, pulmonary alveoli, have two main types of cells.  One cell, type I, absorbs from the air, i.e. gas exchange.  The other, type II, produces surfactants, which help keep the lungs fluid, clean, infection free, etc.  COVID-19 finds a way into a surfactant producing type II cell, and smothers it by reproducing COVID-19 virus within it.  Each type II cell which is killed by the virus causes an extreme reaction in the lungs.  Fluids, pus, and dead cell material flood the lung, causing the coronavirus pulmonary disease.