1. Section of Infectious Diseases, Department of Medicine, Yale University School of Medicine, New Haven, CT 06510.
2. Laboratory of Parasitic Diseases, National Institutes of Health, Bethesda, MD 20892.
3. Department of Cell Biology, Yale University School of Medicine, New Haven, CT 06510.
4. Section of Neurology, Department of Medicine, Dartmouth Medical School, Hanover, NH 03756.