Patients 7 years of age and older with a parasitic infection or condition associated with a parasitic infection (i.e., elevated levels of IgE antibodies or of a type of white cell called eosinophils) who are currently enrolled in a NIH clinical research protocol may be eligible for this study. Relatives of patients and normal healthy volunteers will also be enrolled. Candidates will have a medical history, physical examination and blood tests. Individuals weighing less than 25 kilograms (55 pounds) may not participate.