Before revision:  Here, we use standard tools from statistical physics to account for this effect, which suggest that the rate of appearance of new cases should be proportional to thesquare of the number of susceptible people in the population, not the first power.
After revision:  Here, we use standard tools from statistical physics to account for this effect, which suggest that the rate of appearance of new cases should be proportional to the of the number of susceptible people in the population, not the first power.