54 BULLETIN 780, U. S. DEPARTMENT OF AGRICULTURE. 
cent are infected they become weakened and usually die as a result 
of the infection. Generally speaking, therefore, it may be said that 
when a colony contains less than 10 per cent of Nosema-infected bees 
the prognosis is excellent; that when it contains more than 10 and less 
than 50 per cent the prognosis is fair; that when it contains more 
than 50 per cent the prognosis is unfavorable; and that when the 
number of Nosema-infected bees present approaches 100 per cent the 
prognosis is especially grave. 
In arriving at a decision as to the probable course and outcome of 
the infection the strength of the colony must also be considered. 
This factor, indeed, may be the deciding one. As a rule, the stronger 
the colony, the more favorable is the prognosis. 
In early spring heavy losses among the workers are not replaced 
and the colony weakens. During the active brood-rearing season, 
on the other hand, the bees dying of the infection are replaced by 
young bees. These young bees being free from infection and the 
transmission of the disease within the hive during summer being 
slight as a rule, the prognosis at this season of the year is favorable. 
Experimentally it is found that a single inoculation early in the 
spring will cause a colony to die as a result of the infection produced ; 
if inoculated somewhat later, however, the colony will weaken 
appreciably but will recover from the infection ; if inoculated during 
the active brood-rearing season the weakening effect resulting from 
the infection may not be appreciable; if inoculated toward the close 
of the brood-rearing season the weakness resulting will be noticeable, 
but the colony may winter; and if inoculated later in the autumn 
or during the winter the colony will die as a result of the infection. 
It will be seen, therefore, that the prognosis in Nosema-disease in 
every case is dependent in some measure upon the season of the 
year, being more favorable in the active brood-rearing season than in 
any other. Indeed the season may play a major role in determining 
the course and outcome of the disease. 
The immediate environment of the apiary may possibly play a 
role in determining the prognosis. Opportunity for reinfection from 
without tends to vary the course and outcome of the disease. In this 
connection the nature of the water supply should not be overlooked. 
The extent to which the different races of bees vary in their suscep- 
tibility to the disease, the extent to which individual colenies vary in 
their susceptibility, and the extent to which different strains of 
Nosema apis vary as to their virulence are not at all definitely known 
at the present time. The facts, however, indicate that in no instance 
is the variation particularly great. Much care should be exercised, 
therefore, in ascribing variations in losses from the disease to the two 
phenomena virulence of the germ and resistance of the host. 
