Trematodes parasitize a wide range of hosts, and most species have complex life cycles with alternating sexual and asexual stages. Many trematodes require an intermediate host in which larvae develop before infecting the final host (usually a vertebrate), where the adult worms live. For example, various trematodes that parasitize humans spend part of their lives in snail hosts. Around the world, about 200 million people are infected with trematodes called blood flukes (Schistosoma) and suffer from schistosomiasis, a disease whose symptoms include pain, anemia, and diarrhea. Living within more than one kind of host puts demands on trematodes that free-living animals don’t face. A blood fluke, for instance, must evade the immune systems of both snails and humans. By mimicking the surface proteins of its hosts, the blood fluke creates a partial immunological camouflage for itself. It also releases molecules that manipulate the hosts’ immune systems into tolerating the parasite’s existence. These defenses are so effective that individual blood flukes can survive in humans for more than 40 years.
Urry, Lisa A.. Campbell Biology (p. 694). Pearson Education. Kindle Edition.