i8o 
THOMPSON YATES LABORATORIES REPORT 
As regards Europeans, the fact that few are in continuous residence in badly 
malarial places for long periods renders such questions very difficult to consider. 
Even with negroes there is some reason to suppose that their acquired immunity is 
diminished by residence in a non-malarial country. If in an ordinary attack of 
untreated malaria the parasites be counted in successive cycles, the number is not as 
a rule increased in the second or third observed cycles, and is often diminished. When 
working with Major Ross on Proteosoma, I was able to confirm his observation on 
birds infected by mosquitoes, to the effect that the number of parasites found at first 
rapidly increased, then remained stationary for two or three days, and finally rapidly 
diminished to such an extent that, whilst at the height of the invasion eight or nine 
parasites might be found in a field, later a single parasite only would be found in a 
slide on one day and none on another. 
These observations show that there is a check on the multiplication of the 
parasites, as otherwise they would increase in geometrical progression. Natural cure 
is common, and the parasites disappear or occur in such small numbers that they are 
not found in blood examinations, and leave no deposit of pigment in the organs. 
In many of these cases it is probable that a small residue of parasites is left, 
as in such cases recurrence of the parasites takes place without fresh infection. This 
interval between attacks is one of temporary immunity. It may be long or shorb 
but in newcomers is usually under a month. It varies in different individuals, and 
in the same individual at different times. 
In a place, or even in a house, where frequent fresh infections are occurring, 
the period of immunity may be of several months after a single attack of fever. In 
one station, few newcomers escaped fever for more than a month after arrival ; but 
the older residents did not have attacks more frequently than every three, four, or 
six months or more. In this place, with the older residents, infections must have 
been more frequent than actual attacks of fever ; so that several of the infections 
must have been abortive, indicating a degree of immunity in these persons. 
A common history amongst the older European residents was repeated attacks 
of fever in the first two, three, or four years, and very little or only slight attacks 
subsequently. Many of these persons were not cachetic, and had no splenic enlarge- 
ment. In the temporary periods of immunity the person often appears to be in 
robust health. 
The loss of acquired immunity by change of residence to a non-malarial 
country would, if fully established, indicate that, though immunity to malaria is 
acquired by repeated invasions of the parasite (i.e. relapses), repeated fresh infections, 
however abortive, are required for its maintenance. 
Under these circumstances a high degree of immunity is not likely to be 
attained in countries or districts where for a lengthy portion of each year fresh infec- 
tions do not occur. 
