References
- WHO unpublished document, VBC/75.9.
- Durham , W. F. and Wolfe , H. R. 1962 . Bull. Wld. Health Org. , 26 : 75 – 91 .
- Wolfe , H. R. 1967 . Arch. Environ. Hlth. , 14 : 622 – 633 .
- Copplestone , J. F. 1976 . Bull. World Health Org. , 54 : 217 – 223 .