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In the course of monthly prophylactic treatments with pyrimethamine in a holoendemic malarious area, P. falciparum parasites, apparently resistant to the drug, appeared. During the course of the treatments the resistant varieties spread, replacing sensitive strains : upon cessation of the treatments resistant varieties regressed and became submerged. No spread abroad was detected. Theories concerning the origin of the resistant parasite are discussed.
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Clyde et al. (1954) studied this question.
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