Using vast databases of historical epidemiological data, the scientists tracked the susceptibility of each birth year from 1918 to the present to the different flus in circulation during their lifetime.
科学家借助含海量数据的历史流行病学数据库对从1918年至今出现的不同传播流感的易感性进行了跟踪调查。
Using vast databases of historical epidemiological data -- 动名词作状语
the scientists （主语）tracked（谓语） the susceptibility of each birth year from 1918 to the present to the different flus in circulation during their lifetime.
susceptibility 的主体是 different flus in circulation （在传播中的）
during their lifetime 这里的 their 指 different flus（各种不同流感）
====== 我是英语翻译【查红玉】，希望上述解释对您有所帮助
追问你好 谢谢 请问 from1918 to the present present和theirlifetime是做状语吗 那to the different flu 做什么成分
追答对句子意思和层次清楚，只能告诉你句子意思，语法是我弱项，我也从不学语法，高考英语照样130多分，大学四六级都是大二一次性通过的。
追问好吧 谢谢