  The appearance of the coronavirus (COVID-19) in late 2019 has dominated the
news in the last few months as it developed into a pandemic. In many
mathematics and physics classrooms, instructors are using the time series of
the number of cases to show exponential growth of the infection. In this
manuscript we propose a simple diffusion process as the mode of spreading
infections. This model is less sophisticated than other models in the
literature, but it can capture the exponential growth and it can explain it in
terms of mobility (diffusion constant), population density, and probability of
transmission. Students can change the parameters and determine the growth rate
and predict the total number of cases as a function of time. Students are also
given the opportunity to add other factors that are not considered in the
simple diffusion model.
