2S 
As in our previous reports the cases are tabulated and charted 
according to date of onset of definite symptoms, it having been found 
that in the majority of instances accurate statements as to date of 
onset of the first slight symptoms were not obtainable. 
In view of the generally accepted period of incubation for typhoid 
fever, a period of about 15 to 21 days previous to date of definite 
onset as given probably will represent for the majority of the cases 
the time at which infection occurred. 
If the period of incubation of typhoid fever were constant, epi- 
demiological studies of the disease would be simplified to a great 
extent; but the period seems to be quite variable when compared 
with that of certain other diseases, such as vaccinia, smallpox, 
measles, etc. Furthermore, it is difficult to definitely define the 
incubation period of typhoid fever, as some persons apparently 
begin to have symptoms almost immediately upon receiving the 
infection, while others go at least three weeks or perhaps longer 
before having symptoms. Then there is the period of prodromal 
symptoms, which varies greatly in duration in different cases. So 
that knowing the time that a patient has developed definite symp- 
toms or has taken to bed enables us to estimate but roughly in indi- 
vidual cases just when the infection was contracted. 
It has been observed that when large amounts of concentrated 
infection are ingested, as in milk outbreaks, the period of incubation 
is apparently shortened and in many cases prodromal symptoms are 
absent. It is possible, therefore, that if very small quantities of 
infection are ingested the period of incubation and the period of 
prodromes in some cases may be very much prolonged. 
Judging by observations made in pronounced outbreaks attributed 
to infection in water, the evidence is strongly against the majority of 
the cases having very much prolonged incubation periods. The 
outbreak at Scranton, Pa., reported by Wainwright® offers a striking 
example. There the active period of the epidemic (by report of 
cases) ceased abruptly about 25 days after the infected water supply 
was shut off. Considering the volume of water implicated at Scran- 
ton it does not seem reasonable to suppose that the individuals 
affected there could have received very large doses of infection; 
that is, assuming that the typhoid bacillus does not multiply in 
water under certain conditions. 
The supposition that typhoid bacilli at times may undergo tre- 
mendous multiplication in water under natural conditions is not sup- 
ported by experimental evidence. Of course it is possible that only 
the highly resistant bacilli survive in water, causing pronounced out- 
breaks, and consequently a very small number — perhaps only one — 
of such organisms may in some instances undergo rapid multiplica- 
tion after getting into the alimentary canal of man and cause infection. 
aN. Y. Med. Rec., June 1, 1907. 
