130 Dr Stephens , The Prevention of Malaria. 
fever, and even when present they bear no comparison to the large 
infections with the ring forms or with gametes of native children. 
Thus of twenty-one Europeans who habitually slept without 
mosquito nets, and were so exposed to the constant risk of infection, 
only two shewed parasites and these were extremely rare. Of 
another set of Europeans who used nets and adopted ordinary pre- 
cautions none contained parasites. Again it is a peculiar fact that 
in Europeans who are convalescing from an attack of fever in West 
Africa, gametes are rarely found. This also is the experience of 
Ziemann in Cameroons, so that they are actually not in the 
necessary condition for the infection of mosquitoes. 
These facts then shew that the generally received idea that 
Europeans derive malaria from pre-existing cases in Europeans 
requires considerable modification. This factor it seems to us 
sinks into complete insignificance beside that of infection derived 
from native sources. The normal condition of native children is 
one of almost continuous infection, and there are therefore many 
thousands of cases of malaria in large towns. This enormous source 
of infection has we believe so far entirely escaped recognition. 
Whilst Europeans live in the midst of native quarters exposed to 
infection on all sides, the isolation of such Europeans only as 
have fever is manifestly a futile procedure. 
Malaria then is a contagious disease, the contagion being con- 
veyed by the mosquito, and we deduce as the direct result of these 
observations the conclusion that malaria can be avoided most 
readily by avoiding this source of contagion and by living as far 
removed as possible from native huts. 
Now it is notorious that among men employed on railway 
works in W. Africa malaria is always rife and the explanation is 
clear when we know the conditions under which they live. We 
found without exception in all the railway camps examined by us 
the following conditions. A single European house or a small 
group surrounded by native huts, or with huts in the immediate 
vicinity, and in these huts we always found, as already stated, 
infected Anopheles. 
This dangerous source of infection can be however readily 
avoided by locating the European dwellings at a distance from 
native quarters, and on railways this can be effected with great 
ease. A site then sfiould be selected as far as possible from the 
native villages. We should fix a mile as a perfectly safe limit 
and we shall give reasons for believing that half-a-mile or even 
less would be sufficient. 
We must consider here, as having a direct bearing on this 
question of segregation, the distance of flight of Anopheles, for it 
may be objected that even if segregation be effected there is 
no certainty that infected Anopheles will not fly this distance. 
