Perhaps it was inevitable that New York City and its suburbs would become an epicenter of the coronavirus epidemic in the United States. The population density, reliance on public transportation and constant influx of tourists — all would seem to make the metropolitan area a target.

But to stop the virus, scientists have to figure out which factors played a greater role than others. As it turns out, that is not so simple.

“We have more speculation than facts,” said Dr. Arnold Monto, professor of epidemiology and global health at the University of Michigan.

More than 20,000 New York City residents have tested positive for the virus so far — nearly a third of all confirmed cases in the nation. The prevalence in New York and the areas close to it is far greater than anywhere else in the country.