They had viral pneumonia fatalities and did not identify the lethal virus? They did CT scans on viral pneumonia patients and did not see the pattern?
You make a great point. Yet medical diagnosis is part presumption, part evidence based. If you do not know to look for a disease mechanism, you are unlikely to find it unless you are a research hospital.
As to acquired immunity, I would hope that would be the norm, but like common flu, the strains mutate.