His famous book Wenyi Lun(瘟疫论WēnYì lùn) (means Treatise on Acute Epidemic Febrile Diseases) can be regarded as the main etiological(of or relating to the philosophical study of etiology) work that brought forward the concept, ultimately(you use ultimately to indicate that what you are saying is the most important point in a discussion) attributed to Westerners, of germs(a minute life form (especially a disease-causing bacterium); the term is not in technical use) as a cause of the epidemic diseases. 

The Wu Youke(吴又可Wú Yòukě) (1582-1652 A.D.) — Wu developed the concept(an abstract or general idea inferred or derived from specific instances) that some diseases were caused by the “transmissable agents”, which he called liqi (pestilential factors).