https://blogs.scientificamerican.com/observations/are-crowded-cities-the-reason-for-the-covid-19-pandemic/

Is density really the driver of this epidemic or is the idea misplaced? When people are packed tightly into urban areas they come in contact more often, and thus have more opportunities to spread disease. This intuition coincides with our standard understanding of the spread of infectious disease—but only partly.

To understand how widely and quickly an infectious disease will spread, epidemiologists refer to something called the reproduction number or R. It measures the average number of people an infected person will pass the illness on to over the course of their infection. R is calculated by multiplying four average values: the contact rate of an individual, C; the probability of transmission when a susceptible person contacts an infected person T; the time it takes for an infected person to recover—referred to as the duration D—and the fraction of susceptible people in the population S.