The eggs are eliminated by the urine (in the case of S hematobium) and feces (in the cases of S mansoni and S japonicum) into the water. Under favorable environmental conditions, the eggs hatch within 30 min, releasing a ciliated larvae, called miracidia, that swim and penetrate the specific snail intermediate hosts within 1 to 24 h. Within the snail, the miracidia undergo various transformations with asexual multiplication into primary and secondary saclike structure called sporocytes. The sporocytes migrate to the snail hepatopancreas and start to divide again, producing large numbers of new parasitic forms known as cercariae. Cercariae are 1 mm long and have a characteristic forked tail. Over several weeks, thousands of cercariae may be released from an infected snail, often following a daily circadian rhythm coinciding with the moon or early sunlight, a time in which there is human-water contact activity. In S hematobium, 4 to 8 weeks elapse from the penetration of the miracidia to the liberation of cercariae, whereas in S mansoni, under optimum conditions, only 4 weeks are necessary. The cercariae remain swimming in fresh water, using a whip-like tail, for a maximum of 48 h. They can penetrate the skin of people working or bathing in the infected water in 3 to 5 min using proteolytic enzymes. The most common way of contracting schistosomiasis in developing countries is by direct contact with infected water. Daily activities such as walking, bathing, and swimming or even just foot contact with shallow water may suffice to allow infection to occur. (For a general review, see Lewis,18 Ross et al,24 Mahmoud,25 and Davies and McKerrow26). It is in this context that schistosomiasis constitutes a major health burden worldwide.