<s>[INST] <<SYS>> This is a medical question of type : frequency <</SYS>> How many people are affected by Legg-Calv-Perthes disease ? [/INST] The incidence of Legg-Calv-Perthes disease varies by population. The condition is most common in white populations, in which it affects an estimated 1 to 3 in 20,000 children under age 15. </s>