Read up on the Spanish Flu. Very interesting in how that caronavirus developed and spread versus the normal spread (and why it became so deadly). Interesting note there are multiple strains of the same virus. The deadlier the strain the less likely it is to spread widely - Spanish flu was the exception:
from wiki "....... This increased severity has been attributed to the circumstances of the First World War.[76] In civilian life, natural selection favors a mild strain. Those who get very ill stay home, and those mildly ill continue with their lives, preferentially spreading the mild strain. In the trenches, natural selection was reversed. Soldiers with a mild strain stayed where they were, while the severely ill were sent on crowded trains to crowded field hospitals, spreading the deadlier virus. ........"