Bilharzia infection. Light micrograph of human bladder tissue containing eggs (round, 2 at lower left, 3 at upper right) from Schistosoma flukes. The Schistosoma sp. flukes (small parasitic worms) cause the tropical disease bilharzia (also called schistosomiasis). Eggs are expelled into lakes and rivers in urine from infected humans. The eggs hatch into larvae, infect aquatic snails, and develop into cercariae. After leaving the snail, the cercariae infect humans who enter the water. The mature adult form lives in veins around the bladder, where the females lay many hundreds of eggs that accumulate in body tissues and cause damage.