Good question. I am making the case for early origin and critical mass, not unlike an atomic bomb’s initial explosive stages. Radiation fallout is another essay.

This is a little difficult to grasp as a scientific concept, but infectivity or R0 is not a constant for a virus like the gravitational constant or pi. If you have a population which regularly spits, then R0 goes up. If the population regularly attends church, confined to a nursing home, R0 is likewise higher. If a large portion of the population smokes, or lives with air pollution which damages lungs, or increases the number of binding sites in the lung for the virus, then R0 skyrockets.

If a population regularly washes their hands, then R0 goes down. If they drive individual cars instead taking of mass transit, then R0 goes down. If infectivity is calculated under temperature and humidity unfavorable for viral survival and spread, then for that snapshot in time, you will get different numbers.

In other words, infectivity is not simply related to the infectious organism, but has societal factors, personal hygiene as well as time frame measurement factors including weather factors such as humidity and temperature.

In other for an epidemic to get going, you need a perfect storm. A critical number of initially infected people, animals bred on large farms, cages stacked on each other, a large group of susceptible people made possible by hygiene practices, large open emergency rooms, ongoing favorable weather, etc. Lots of viruses never get this far.

Once they pass the initial stages, it becomes relatively easier to explode.