Soil-transmitted helminth infections and physical fitness in school-aged Bulang children in southwest China: results from a cross-sectional survey oleh Yap Peiling, Du Zun-Wei, Chen Ran, Zhang Li-Ping, Wu Fang-Wei, Wang Jian, Wang Xue-Zhong, Zhou Hui, Zhou Xiao-Nong, Utzinger Jürg, Steinmann Peter