Helminthology is the study of parasitic worms (helminths), while helminthiasis describes the medical condition of being infected with helminths. Helminthology deals with the study of the taxonomy of helminth and the effect on their hosts. The origin of the first compound of the word is from the Greek ἕλμινς - helmins (root: helminth-), meaning "worm". The Japanese parasitologist Satyu Yamaguti was probably one of the most active helminthologists of the 20th century; he wrote a series of six volumes entitled "Systema Helminthum".