Complex sentence: Schistosomiasis is a major tropical disease, second only to malaria, caused by two species of fluke worm "Schistosoma", infecting some 200 million people in 74 countries.
Split into simpler sentences: Schistosomiasis is a major tropical disease, second only to malaria. It is caused by two species of fluke worm, "Schistosoma", infecting some 200 million people in 74 countries.