The basics of epidemic spread

  • The COVID-19 outbreak that started in January 2020 spread rapidly, first in Wuhan, China, and then around the world. Rapid disease outbreak confined to a particular region is an epidemic. A pandemic is an epidemic that spreads globally. COVID-19 was declared a pandemic on 11 March 2020 by the World Health Organization
  • We study epidemic spread using what is called an S-I-R model, where S stands for 'susceptible' (individuals that can be infected), I for 'infected' (individuals with the virus), and R for 'recovered' (those that have had the disease and gotten better.
  • We can make the models more realistic, and complicated, by adding other categories (like those who were infected and died) but here I present just the simplest model as it effectively demonstrates the time-course of an epidemic
  • We model the flow of people between these three states, assuming complete mixing in each category, no intermediate vectors (like a mosquito), and that recovered individuals are immune to the infection as follows: