Somewhere in the middle may be schistosomiasis, a debilitating disease caused by a parasitic flatworm carried by water snails, which affects 200m people worldwide.

• 居于这些层次中间的可能是血吸虫病，一种由寄生于钉螺的扁平虫引起的影响患者健康的疾病，它影响着世界上2亿左右的人口。