4
increased endemicity, we shall term for descriptive purposes the
FACTOR OF NON-IMMUNE IMMIGRATION.
But non-immune immigration does not alone constitute the whole
of the human factor in malaria.
Celli in a recent reference to malaria in Italy has corrected the
popular notions that the Roman Campagna is an undrained marshy
tract, and that malaria there is due to want of drainage. He believes,
on the contrary, that the causes at work in producing the intense
malaria of the Roman Campagna are in the main bound up with the
social and economic condition of the people, and especially with the
hardships that the labourers have to undergo while working in the
Campagna under a system which leaves them largely in the
hands of the headmen and labour contractors, a statement singularly
in accordance with what we shall hereafter find it necessary to make
in regard to the Duars.
As regards India, Stephens and Christophers have called atten-
tion to a feature of malaria as observed by them in this country, namely
that among different classes living in the same locality under very
similar conditions as regards exposure to malaral infection there exist-
ed a greater prevalence of malaria among those of low social status
than among those of a higher level. It is unnecessary to go further
into this question beyond stating that among aboriginal tribes and
certain poverty-stricken communities in India a very high endemic
index of malaria may frequently be found, depending apparently upon
the general squalor, the hand-to-mouth existence, and other conditions
associated with low social status.
The way in which such economic conditions influence malaria
seems to be in bringing about relapses and a state of continued infection.
Ross, in computing the probable time during which infection may
remain in the body capable at any time of being stimulated into ac-
tivity by the action of depressing influences, fixes the period in British
troops at about six months. In this case he was dealing with Eu-
ropeans, originally healthy, picked men, well-fed, well-housed, and,
under constant medical supervision. But, when we consider natives,