Acknowledgments. We gratefully acknowledge the advice and co-operation of Dr. V. P. Sharma and Dr. S. K. Ghosh in ®eld studies and the help of Dr. W. E. Collins and B. Holloway. This work was supported in part by the USAID PASA BST-0453-P-HC-2086-07 grant administered in India by a CD and RI program of the Department of Biotechnology;
and an USAID/NIAID/CDC intraagency agreement Y-02-AI-20004-021;