The disease was rare at the beginning of the century, and worldwide there were only a few thousand cases per year, but by the 1950s there were 60,000 cases each year in the US alone and an average of 2,300 in England and Wales.
Eelmise sajandi alguses oli haigus haruldane ja aastas esines vaid mõni tuhat juhtu, aga 1950-ndateks oli ainult Ameerika Ühendriikides igal aastal 60 000 haigusjuhtu ning Inglismaal ja Walesis 2300 juhtu.