3
camps is, we believe, not dependent upon the number of anopheles
(a certain number of which are always present), but on the human
factor.
It is desirable before going further to see what is known in regard
to this much neglected human factor in malaria; for it is obvious that,
if in such diseases as cholera and typhoid we were to be ignorant of,
or neglect, the part played by human disseminators, we should have
a very imperfect notion of the mechanism at work in the spread of
these diseases, though we might know well enough that they are con-
tracted by the swallowing of particular germs.
Koch investigating malaria at Stephensort described three types
of villages:-
(1) Villages with little or no malaria.
(2) Villages with malaria in considerable amount, but confined
almost entirely to the young children.
(3) Villages where malaria was prevalent, not only among
young children, but among the general adult population
as well.
Examination showed that the population of the third type of
village, instead of being fixed or permanent, as in the case of villages
of types 1 and 2, was subject to fluctuation owing to causes which led
to the introduction of non-immune immigrants. At each introduction
of such people a sudden burst of increased malarial infection occurred,
lessening as this new population became partially immune by longer
residence, but showing recrudescences whenever a fresh immigration
took place.
A condition of continual immigration similar to that described by
Koch is conspicuously present in the Duars, Assam and elsewhere:
in India, where the constant introduction of non-immune immigrants
may be likened to the continual heaping of fresh fuel upon an already
glowing fire.
This factor, which when it acts temporarily, is capable of produc-
ing epidemic malaria and when long, continued must give rise to an
A2.