The purpose of this study is to determine what causes some people to become sick, and others not, when they are infected with the parasite Schistosoma haematobium, also known as Bilharzia. This is an infection of the urinary tract blood vessels and can cause serious disease. Approximately 4400 adults and children of any age will participate in this study. They must be residents of the Msambweni Area, Kwale District, Coast Province, Kenya, where infection with S. haematobium parasites are common. To find out if people are infected, they will first provide 1 or more urine samples for a microscope examination to detect if the S. haematobium parasites are present in the body. Volunteers then will be examined by ultrasound to see if they have kidney or bladder disease. (Ultrasound examination is the use of a non-painful machine that uses sound waves to examine the condition of the internal organs.) Treatment with the drug praziquantel will be offered if S. haematobium infection is found.