A number of studies have reported that
Plasmodium species apparently suppressed each other, in population [
1,
2], or in a mixedly-infected individual [
3‐
8]. It is noteworthy that the emergence of
Plasmodium vivax in patients' peripheral blood has led to a "total disappearance" of
Plasmodium falciparum, while passive transfer of
P. falciparum-immune IgG exhibited weaker suppressive effects on
P. falciparum in these patients [
9]. The severity of
P. falciparum infection has been reported to be dramatically ameliorated in patients simultaneously infected with
P. vivax [
2,
10]. Mutual suppression between
Plasmodium parasites has been thoroughly reviewed [
11‐
13]. These observations have led to speculation that the much lower disease-specific mortality and case fatality rate from malaria in Asia-Pacific region than in Africa may be due to the presence of so-called "benign"
P. vivax malaria [
14,
15], because
P. falciparum is by far the most dominant in sub-Saharan Africa [
16]. Therefore, understanding the interaction of these
Plasmodium species is important, especially because several
P. vivax vaccines are currently being developed. The authors hypothesized that the host immunological reactions to infecting
P. vivax might be playing a role in this suppression of co-infecting
P. falciparum. Although such a notion has been long held [
17‐
19], actual data has been rarely available for human malaria. However, any study design that leaves ailing human volunteers (other than the researcher) untreated is unacceptable from a bioethical point of view. By contrast, "almost all of the associated legal, ethical, and metaphysical problems vanish" with self-recruitment of the researcher, according to the founder of modern bioethics [
20]. Hence, to examine the hypothesis, YN volunteered to be infected with
P. vivax, and his serum was collected sequentially during this infection. The inhibitory effect of these sera upon
P. falciparum growth was quantified by using an
in vitro culture technique. To illustrate the kinetics of the immunological processes, immunological mediators including cross-reactive antibodies against
P. falciparum and cytokines in these sera were also measured.