Participants will have a physical examination and laboratory tests on blood, stool, or urine. Blood samples may be collected at regular intervals, but no more than 450 ml (15 ounces) of blood will be drawn from adults, and no more than 7 ml (1-1/2 teaspoons) per kg (2.2 pounds) of body weight from children, in any 6-week period. Other tests may include x-rays, electrocardiogram (EKG), or tissue biopsy (surgical removal of a small tissue sample), depending on the individual s condition.