MALARIA AND THE COMMUNITY 
153 
size and distribution, and is favored or 
limited by a variety of factors. This, how¬ 
ever, results in a certain transmission rate 
which determines the character of the ma¬ 
laria in the community. At every level, 
this transmission rate is attended by a cor¬ 
responding tolerance made up of individual 
specific reactions to plasmodial infection, 
and at high intensities, the group immunity 
intervenes forcefully to protect the popu¬ 
lation. It pushes the acute struggle back 
to the earlier ages, and when completely 
established, restricts it to the first two years 
of life. Such intensely malarious com¬ 
munities do not usually increase in num¬ 
bers, and the adults are able to live a rela¬ 
tively normal and active life. 
Chronic Cases, Mixed Infections and 
Parasite Formulas 
It is known (through induced malaria in 
paretics) that the immunity conferred by 
infection is solid and lasting in the case of 
P. vivax; solid also, but less easily acquired 
and more evanescent in P. falciparum. P. 
vivax immunity may last as long as 7 years, 
and that to P. falciparum several months 
at least, sufficient to cover the ordinary 
transmission season. It is clear that no 
one could develop chronic malaria by re¬ 
peated infection with any one parasite. 
Persistent splenic enlargement is not pro¬ 
duced in the laboratory even with several 
strains. James repeatedly reinfected one 
of his cases over a period of 5 years, first 
with P. vivax (7 times), then with P. falci¬ 
parum, then again with P. vivax and finally 
with P. ovale without producing anything 
but a high resistance to all three parasites. 
But this immunity is very specific. A 
person acquires immunity to the particular 
strain employed and not to any other strain. 
There is a slight spread of tolerance in P. 
vivax infections, but hardly any in P. falci¬ 
parum. As for different species, it has 
been shown that a current or recent infec¬ 
tion with either P. falciparum or P. vivax 
parasites in no way interferes with success¬ 
ful inoculation with the other. 
Chronic malaria, then, is due to over¬ 
lapping infections of different species and 
heterologous strains of plasmodia. Mixed 
infections must be the rule and not the ex¬ 
ception in localities with even a moderate 
transmission rate. Christophers has pointed 
out that there will be overlapping infec¬ 
tions even with relatively low endemicity, 
for if one hundred infections were distrib¬ 
uted at random among a hundred individ¬ 
uals, the chances are that: 
37 would escape infection. 
37 would get 1 infection. 
18 “ “2 infections. 
6 “ “3 “ 
2 “ “ 4 or more infections. 
There is thus being formed a nucleus of 
individuals who have been infected twice 
or more, and in fact 26 per cent will already 
have mixed infections. 
It is well known that these mixed infec¬ 
tions are not found in the ordinary micro¬ 
scopic survey of blood specimens. Natu¬ 
rally we should not expect to recognize 
strains, but the species ought certainly to 
show up. Yet mixed infections are re¬ 
ported as present in only 2 or 3 per cent 
at most of the preparations. There is an 
antagonism between the three common spe¬ 
cies which ordinarily enables only one of 
them to remain in the circulating blood in 
sufficient number to be discovered by rou¬ 
tine examination. It seems that P. falci¬ 
parum usually commands during the acute 
stage, P. vivax tends to dominate the long 
term relapses, while P. malariae, a tenaci¬ 
ous parasite, outlasts them both. 
The only way to obtain an approximate 
idea of the percentage of mixed infections 
in a population is to follow a group through 
the year with persistent blood examina¬ 
tions. The proportion found will natu¬ 
rally correspond to the intensity of the 
malaria. Barber and Komp found 45 per 
cent of a group of children in the southern 
United States varying in type of plasmo- 
dium during the year, and Balfour re¬ 
ported the same percentage in a Gfcreek vil¬ 
lage, although not more than 3 per cent of 
mixed infections were found at any one ex¬ 
amination. We conclude that children can 
not grow up in a malarious locality of even 
moderate endemicity without acquiring a 
