求英语高手分析一句中级口译课本里的一句句子

求英语高手分析一句中级口译课本里的一句句子句子如下:it will cause such a significant amount of disease in the middle ages of the population that it will largely reduce the mumber of people available to carry out the functions of the society.这句句子面为什么that 后面还要加it,为什么不直接that will-----,请高手分析下整句句子,