As of today, most countries are on lockdown; a strategy devised by governments to help slow down the spread of the novel coronavirus COVID-19. Moreover, officials use the term flattening the curve to help the healthcare system cope with the expected large amount of patients, but what does that mean mathematically? I will go on an overview of how to construct a useful toy mathematical model to get a qualitative view of the dynamics of the virus. This is a typical SIR model appearing a lot lately in the media. I will go further to explain how to develop such a model and explain the power and limitations of such models. How to construct SIR models SIR stands for Susceptible, Infectious and Recovered (or Removed) agents. In our example, the agents are humans spreading the disease to each other. To simplify things, we model an agent as a node and the links between nodes represent social connections, as seen in the picture below: Figure 1: Modelling humans as nodes