, can cause extremely severe poisoning. But this is not the (54) ______ problem. The (55) ______ illness or death of farmers, farm workers, and others exposed to sufficient quantities of pesticides are very sad and should not occur. For the population as a whole, we must be more concerned (56) ______ the delayed effects of absorbing small amounts of the pesticides that invisibly pollute our world. 　　Responsible public health officials have (57) ______ that the biological effects of chemicals are cumulative over (58) ______ periods of time, and that the danger (59) ______ the individual may depend on the sum of the exposures received throughout his lifetime. For these very reasons the danger is easily (60) ______. It is human nature to shake off what may seem to us a (61) ______ of future disease. ""Men are naturally most (62) ______ by diseases which have (63) ______ signs," says a wise physician, Dr. Rene Dubos, ""(64) ______ some of their worst enemies (65) ______ approach them unnoticed."" --------------------------------------------------------------------------------