A tuberculin skin test survey utilizing standard human PPD and PPD's prepared from some unclassified mycobacteria was conducted in 2,074 school children in Kuala Lumpur, Malaya. Results of the survey are consistent with the hypothesis that 'non-specific" agents are prevalent and acting there. Sensitivity to the Battey PPD was very high in the population tested and there was some evidence of cross sensitivity to human PPD. The public health implications of this are discussed. On the basis of the local findings, a suggestion is made to change the local criterion of a negative tuberculin reaction, for the purpose of giving BCG vaccination, from less than five mm of induration to less than nine mm of induration.
The particular agricultural adaptation we have been considering is the ultimate determinant of the presence of malaria parasites in the intracellular environment of the human red blood cell. This change in the cellular environment is deleterious for normal individuals, but individuals with the sickle-cell gene are capable of changing their red-cell environment so that intense parasitism never develops. Normal individuals suffer higher mortality rates and lower fertility rates in a malarious environment than individuals with the sickle-cell trait do, so the latter contribute proportionately more people to succeeding generations.