Book Review: The Rules of Contagion by Adam Kucharski
Published/Updated: October 31 2026
I really like these kinds of books that set up a central premise that is later applied to different case studies. There are many people out there that have written better reviews for this book, so I just wanted to mention the main takeaways I got.
The most important part of this book is the one that describes R:
R = duration * opportunities * transmission probability * susceptibility.
Reproduction Number (R): number of new infections caused by an individual
Duration: duration of infection for an individual
Opportunities: # of chances to spread an infection each day
Transmission Probability: probability an opportunity leads to an infection
Susceptibility: average susceptibility of the population to an infection
Reproduction numbers can be used to determine how many people need to be vaccinated for herd immunity to occur. Herd immunity is the percentage of a population that needs to be vaccinated that lowers the reproduction number to 1. A reproduction number of 1 means that each infected person infects only one other person on average. Assuming a vaccine is 100% effective at preventing infection, the following equation solves for herd immunity:
Final R = Initial R * (1 – P)
Final R: The target R value (we want 1 or below)
Initial R: The R value of the infection
1: The percentage of susceptible people
P: The percentage of immune people
One example given by the author is measles with an R value of 20 (initial R). We want to know P when Final R is equal to 1. Therefore, we can solve the formula like this:
1 = 20 * (1 – P)
P = 0.95
We need 95% of the population immune for the R value of measles to be equal to 1.
When R is less than 1, we can use the following formula to determine how many cases will result from infection. This is important information to know as infections can adapt and increase their reproduction number:
cases = 1/(1-R) | 0 < R < 1
With R taken care of, the concept of networks is crucial to understanding disease transmission. The idea is that people exist in their own special circles. For example, if I have COVID-19 I probably won’t directly infect you, the reader, unless you were in my network. There are different classifications of networks that influence how quickly and how far an infection can spread (assortative/disassortative).
Outside of biology, Kucharski uses the rules of contagions to talk about polarizing topics such as violence, moral beliefs, misinformation, and internet hype. Their perspective is very well appreciated and I found myself already thinking about these topic even before I read this book. I highly recommend checking this book out if your looking for an easy and fun read!