That study also found that the mean time between incidents was 1.02 years, while for Beth in the book, it’s mentioned as being closer to three years later, although she never seems to be fully better after her first bout with the fever.
A recent study of 158 patients who had scarlet fever found that 16 individuals suffered from more than one episode.
That may not seem like a lot, but it is 10% of people who end up contracting the disease.