Like other trematodes, Schistosoma spp. have very complex live cycles involving multiple hosts. Here's a quick overview image:
So: the schistosome's life cycle. They start out as little eggs which leave the adult's host (humans) through the waste systems in urine or feces. These eggs then hatch into the first larval stage, which is called a miracidium. The miracidium seeks out a snail and burrows through its tissues to get to its gonads. There, it asexually reproduces to create 'sporocysts', and in the process thousands of copies of itself while castrating the unlucky mollusk. When the time is right, these copies transform again into cercaria, which kind of look like sperm, and for the same function: swimming. Upon bursting out of the snail, these little swimmers hunt down the nearest suitable host they can find by actually swimming towards them.
Now here's the sci-fi and kind of gross part. The cercaria, upon finding a nice new home, secretes an enzyme that breaks down our skin and burrows in deep. Soon enough, it finds a capillary and travels via our blood to our liver. There, they mature into adults, gorging themselves on red blood cells and finding mates to have casual encounters leading to lots more eggs.
And, like so often with parasites, it's the eggs that are the worst part. Some species of Schistosoma can produce thousands of eggs a day. These eggs aim ultimately to pass through the intestinal walls or the urinary tract and leave in feces or urine, but most fail to do so. Those that fail wreak havoc on our bodies. Because of enzyme secretions by the eggs, our immune system launches a devastating attack which ends up harming us more than the invading parasite young.
The disease state of being infected, called schistosomiasis, is awful, and more than 200 million people are infected worldwide every year, 300,000 of which die from it. It's a chronic infection which is a big problem in some third world countries where easy access to clean bathing waters and trematode-fighting drugs are unheard of. Infected individuals experience abdominal pain, cough, diarrhea, fever, fatigue, anemia, enlarged spleen and liver and generally feeling like crap. If untreated, eggs can end up in the nervous tissues and the brain, where they can cause seizures, paralysis, or spinal cord inflammation. Children who are repeatedly infected can develop anemia, malnutrition, and learning difficulties.
But, far worse than all of that is the calcified granulomas. Our bodies, in response to the invasive parasite, tell our immune system to attack at full force where the trematode's eggs end up. Over time, these damage tissues can take in calcium and become hardened scar tissue. The more eggs and the worse the body's reaction, the more hardened tissue forms. The chronically infected end up with large, hardened masses of liver and bladder, which, eventually, are fatal.
The sad part is that treatment is easy and safe, but so many of those who are infected have no access to it. The drug of choice is praziquantel, and each tablet costs approximately $0.08. To treat a child, which takes a few tablets, costs $0.20. Even still, many in the heavily infected areas cannot afford to keep taking the medication, which, by killing the worms, prevents the liver damage from getting bad enough to look like the children above.
Scientists are working on a vaccine for the elusive worm, but as with trying to vaccinate against any metazoan parasite, it's tough to find something that doesn't change to avoid immune detection. In the meantime, other measures include understanding which waters are infected, when the cercaria are most active, and getting villages safe, clean drinking and bathing water so they don't have to end up wading in trematode-infested waters.