56 
Table 1. — Period of incubation in-yellow fever, etc. —Continued. 
[Francis and Beyer.] 
Case No. — 
Bitten. 
Attack. 
Incubation. 
40 .. 
Sept. 11, 9 a. m.,and 
Sept. t2, 2.30 p. m. 
Sept. 14, 3.30 p. m_ 
3 days 7 hours, or 2 days 1 hour. 
A study of the 40 cases in this table discloses the fact that yellow 
fever usually begins about three days after the mosquito bites. 
The period of incubation resulting from this natural method of 
conveying the disease is rarely under three days. We have but one 
such authentic instance, namely, two days twenty-four hours (case 
No. 24). 
The longest period observed was seven days five hours, but it must 
be noted that the man who had this unusually long period of 
incubation had previously been treated with injections of immunizing 
sera, which may have delayed the onset and modified the disease, 
for he had a mild attack. 
Leaving this case (No. 39) out of consideration, the longest period 
of incubation resulting from the bites of mosquitoes is the case (No. 
14) of Reed, Carroll, Argamonte, and Lazear, in which an incubation 
period of six days two hours was observed. This corresponds strik¬ 
ingly to Carter’s clinical observations in which he reports a case with 
an incubation period of five and three-fourths days. See case No. 12 
in Table 1. 
The French commission, working in Rio de Janeiro, came to the 
concluson that the period of incubation of the disease may be much 
longer than this; but we find on analyzing their ivork that they dreAV 
their inferences largely from the disease produced by such artificial 
means as the inoculation of modified blood serum. 
One of the conclusions of this commission was that yellow fever 
may not infrequently incubate for twelve days before symptoms 
declare themselves. 
They state that “ this incubation of twelve days is not absolutely 
rare. We have had occasion to see that the natural infection may 
also present an incubation equally long.” 
With this statement we must take issue, for the long experience 
of the Public Health and Marine-Hospital Service in the many 
wars it has waged against yellow fever has amply demonstrated 
that for practical purposes five days is sufficient to cover the period 
of incubation of the great majority of cases. An analysis of all the 
cases reported in Table 1 supports this view. 
The French Commission reports several cases in support of their 
contention. One, a young man 18 years old, who took yellow fever 
ten days after having arrAed in Petropolis from Rio de Janeiro. 
Petropolis is a village free from yellow fever. Another instance 
