55
Paras. 29—30
certitude, but have found it impossible to do so in the cool season in time for
this report.
According to many writers malaria is one of those diseases in which
individual immunity is not developed. I think that this is a mistake ; and a
mistake which has risen from confounding the ordinary recrudescences of fever
with fresh infections—it is thought that each recrudescence is due to a new
invasion, and it is therefore naturally inferred that the preceding attack has
conferred no immunity. This is a wrong conception of a malarial infection.
Such an infection, if left to itself, normally develops successive recrudescences
which are only a part of the original invasion. Looking at the subject in this
light, I think we are bound to admit that temporary individual immunity may
be finally established, either after an extremely severe single attack or after a
long pervasion of the blood by the parasites. It is difficult to see how, apart
from treatment, recovery can ever occur without it.
30. Conditions favourable to the Early Extinction of the Para-
sites.—Since it is possible for the invasion to become extinct after a certain
lapse of time, we may now enquire the conditions most favourable to this. I
have already suggested an answer. In paludism, as in other diseases, the energy
of resistance is likely to vary directly as the morbific energy of the invasion.
In other words, the more severe the initial infection, the more vigorous must be
the reaction if death is not to ensue.
The system finds itself confronted with a life or death struggle. All its
powers are roused ; either the host or the parasites must perish.
In a mild attack the forces of defence are not sufficiently roused—the
invasion is repulsed, but not extinguished. In a severe infection, when very
existence is threatened, they are roused to the utmost ; if they be successful the
invasion is not only repulsed, but the impetus of the defence suffices to destroy
the invaders completely, or very nearly so.
We have been made quite familiar with this fact by the multitude of
experiments for the preparation of antitoxin and antivenine. Unless an
animal be treated with a sufficiency of the toxin or venin, its blood does not
produce a sufficiency of the antidotes.
This result may seem paradoxical. The general experience is that the
worse the infection the less are the chances of recovery. Precisely ; but we are
forgetting the condition always implied by those theories. I say, if the defence
be successful it will be more likely Ultimately to extinguish the invasion in a
severe than in a mild attack. Of course, in the former, before the system is fully
roused to its danger, the parasites are more likely to win the victory immediate-
ly, the defence is less likely to be immediately successful, death forthwith is
more probable. Again, we must not forget that the parasites produce a secondary
effect on the organs which must, cæteris paribus, vary in degree directly as the
violence of the infection, and which may cause death ultimately after the actual
extinction of the parasites themselves. These various issues must be carefully
kept apart.
What are the actual conditions, then, which are most favourable to the early
establishment of immunity ? The virulence of the parasite, the numerosity
and repetition of the infection, and the absence of treatment.
It will be seen at once that all those conditions are exactly those which
obtain among aboriginal populations living in very malarious localities. They
are subject to virulent parasites ; the number of parasites acquired in one infection
is likely to be greater in a very malarious locality than in a less malarious one ; the
chances of repeated infection are greater ; while such patients seldom take quinine.
The patients are not necessarily infected on one occasion only, but possibly
over and over again for days or weeks. They are even possibly open to re-infec-
tion from themselves. To take an illustration from proteosoma of birds—I have
just stated that sparrows were infected and re-infected by mosquitoes. Now,
mosquitoes do not bite only once or twice ; nor do they live only for a few days.
They may certainly live for a month, probably in the same house or tree, and
will continue to feed themselves every two days or so, according to the tempera-
ture. The same bird may become infected over and over again by the same
mosquito. If the bird has an original infection, the mosquito may become
infected from it, and after a week or so may infect it again with a worse infection.
In this manner the bird may, so to speak, re-infect itself, by the mediation of the