They're referring to the Markov chain model as the "Swiss cheese" model, which isn't the best name, considering it's connotations in sports.

https://www.nytimes.com/2020/12/05/health/coronavirus-swiss-cheese-infection-mackay.html?smid=tw-share

![](https://static01.nyt.com/images/2020/12/04/science/00SCI-VIRUS-CHEESE2/merlin_180804873_4914b9d4-f0f0-4891-aad4-cb927403d8bc-jumbo.jpg)