The Dielmo project, initiated in 1990, consisted of long-term investigations on host-parasite relationships and the mechanisms of protective immunity in the 247 residents of a Senegalese village in which malaria is holoendemic. Anopheles gambiae s. 1. and An. funestus constituted more than 98% of 11,685 anophelines collected and were present all year round. Inoculation rates of Plasmodium falciparum, P. malariae, and P. ovale averaged respectively 0.51, 0.10, and 0.04 infective bites per person per night. During a four-month period of intensive parasitologic and clinical monitoring, Plasmodium falciparum, P. malariae, and P. ovale were observed in 72.0%, 21.1% and 6.0%, respectively, of the 8,539 thick smears examined. Individual longitudinal data revealed that 98.6% of the villagers harbored trophozoites of P. falciparum at least once during the period of the study. Infections by P. malariae and P. ovale were both observed in individuals of all age groups and their cumulative prevalences reached 50.5% and 40.3%, Î-espectively. Malaria was responsible for 162 (60.9%) of 266 febrile episodes; 159 of these attacks were due to P. falciparum, three to P. ovale, and none to P. malariae. The incidence of malaria attacks was 40 times higher in children 0-4 years of age than in adults more than 40 years old. Our findings suggest that sterile immunity and clinical protection are never fully achieved in humans continuously exposed since birth to intense transmission.

Despite uncertainty as to the accuracy and reliability of the methods used to measure malaria morbidity and mortality,'" epidemiologic studies in regions of high malaria endemicity have consistently shown that the severity of the disease decreases considerably after the first years of life. Deaths from malaria occur mainly among infants and young children," 5 and a marked decrease in the incidence of clinical attacks is observed after five years of age. 6 In contrast, parasite prevalence and incidence remain high throughout adolescence and only decrease slowly in adults. 7. s This nonsterilizing protective immunity has been progressively recognized since the historic studies of Chfistophers in the 1 9 2 0~.~ However, despite decades of research, knowledge of how parasitization results in disease is incomplete and no clear picture of the mechanisms of naturally acquired immunity to malaria has yet emerged. I0-'*