Answer: Because some mutations can exert selection pressure on some mutations, selecting them positively in order to perpetuate the species.
Explanation:
Malaria may have exerted selection pressure on the human genome, causing it to evolve by selecting those genes that provide this resistance to the Malaria disease, prevailing against the negative effect of the mutation itself. Therefore, this phenomenon is seen in populations where mutation of the gene that codes for glucose 6-phosphate dehydrogenase is common, for example in some african pupulations.