Due to a series of factors, a very low proportion of carriers (less than 1%) will eventually become ill. This most frequently occurs in young children, but a second disease peak is observed among adolescents and young adults.
Zaradi številnih dejavnikov zboli zelo majhen delež prenašalcev (manj kot 1 %). Najpogosteje zbolijo majhni otroci, drugi vrhunec pogostosti pa bolezen doseže med mladostniki in mladimi odraslimi.