Flukes generally have life cycles with an alternation of sexual and asexual stages. Many species require an intermediate host where larvae develop before infecting the final host, where the adult fluke lives. For example, flukes that parasitize humans spend parts of their life histories in snails (FIGURE 29.15). The 200 million people around the world who are infected with blood flukes (Schistosoma) suffer body pains, anemia, and dysentery.