Given the spread of the Covid-19, here's how to explain the mathematics of epidemics. "The 'basic reproduction number' R0 is defined as the expected number of secondary cases produced by a single infection in a completely susceptible population. It is important to note that R0 is a dimensionless number and not a rate." R0 is the largest eigenvalue of the 'next generation matrix'. For example, R0=phi=(1+sqrt(5))/2 for Fibonacci's rabbit problem; R0='k' (neutron multiplication factor) in nuclear fission. Thus, R0 is the *base* of the exponentials & logarithms which occur in the various solutions instead of Euler's 'e'. Clearly, an epidemic won't go 'viral' unless |R0|>1. "Notes on R0" James Holland Jones, Stanford University, 5/1/2007 https://web.stanford.edu/~jhj1/teachingdocs/Jones-on-R0.pdf