It’s an old idea, you’d basically have a sleepover of uninfected individuals with a few infected individuals. The idea was you’d give everyone the disease at the same time, meaning the next time the disease came through you’d have unified immunity, rather than a mix some individuals being immune and some still being susceptible having avoided infection the last time. It’s better than nothing for disease control, but only just barely.

Of course, it became unnecessary with vaccines for the various diseases “treated” that way. Since with a vaccine you can gain immunity without actually having to catch the disease in question.