Here is my self-satisfactory and yet mathematical understanding on how COVID-19 spreads and disappears. This is not specific to COVID-19, but it is general.
Let me assume that the number of infected persons per area n increases proportionally to the number of contacts per area per time Z between infected persons and potentially infected persons while it decreases proportionally to n. So the basic equation is written as
When people moves randomly like gas molecules [1], Z is proportional to n and the number of potentially infected persons n’. It is probably not so arrogant to assume n’ as N-n-V…
