Light micrograph of a dead Schistosoma mansoni trematode egg. The eggs are produced by female flukes living in the veins of the intestines and bladder of humans. They travel to rivers and lakes via untreated sewage. The larvae hatch and infect water snails, leaving these hosts as free-swimming cercariae. These can penetrate the skin of anyone in the water, beginning a new cycle of human infection. Bilharzia is endemic in tropical countries, and can cause kidney and liver damage.