Something is odd about this virus. It either runs a mild course or puts fit and young (as well as old and comorbid) people onto a ventilator with seemingly irreversible progression to lung failure and death. Why does the pneumonia grip so severely in one group and doesn't arise in the other? There must be multiple strains. With influenza (a different family of viruses), it is predictable in that it kills the old and comorbid but spares the young and fit - it follows a course that is explainable and predictable. The covid19 is killing lots of old for sure, but teenagers are dying in Europe, doctors are dying everywhere. It seems to be randomly severe. This can only be explained if there are multiple strains in action simultaneously, with varying potency.