The parasitic worm that causes schistosomiasis hatches in water and grows up in a freshwater snail, as shown here.
Once mature, the worms swim back into the water, where they can infect people through skin contact. Initially, an infected person might have a rash, itchy skin or flu-like symptoms, but the real damage is done over time to internal organs.
The image above shows a slice through the tentacle of an infected snail. The swirls of orange, blue, and green on the periphery of the image are snail muscle. The darker centre of the image reveals a mother worm in which hundreds of daughter worms are developing. The daughters will leave the mother, migrate, and colonize other parts of the snail, and ultimately give rise to thousands even millions of individuals that can infect humans once they are released into fresh water.
This massive multiplication inside the snail maximizes the transmission of this major human parasite.