1. Department of Pathobiology, Johns Hopkins University School of Hygiene and Public Health, Baltimore, Maryland 21205
2. Division of Parasitology, Calcutta School of Tropical Medicine, Calcutta 12, India
3. Johns Hopkins University Center for Medical Research and Training, Baltimore, Maryland 21205
4. Department of Biostatistics, Johns Hopkins University School of Hygiene and Public Health, Baltimore, Maryland 21205