24 
V arying degrees of vimlence (and infectiveness) of different strains 
of typhoid bacilli may be the explanation of some of the pecuhar 
features of typhoid fever observed in epidemiological studies. 
It may be that in communities where typhoid fever prevails endem- 
ically the period of incubation varies within much wider limits than 
is the case in epidemics when a large number of persons get the 
infection from a common source, and all presumably from the same 
strain of organism. 
The striking seasonal prevalence of tj^phoid fever observed in 
Washington and in many other endemic foci may possibly find its ex- 
planation as follows: The infectious organisms from various sources 
disseminated through the community by water, milk, raw shellfish, 
personal contact, flies, salads, etc., during the winter and spring 
months are, when ingested by persons, incapable of causing the 
disease for the majority of the cases at that time; but they become 
stored in the bodies of these persons and with the advent of warm 
weather other factors become operative and disease results. 
It is known that persons may carry the typhoid bacillus for years 
without clinical evidence of typhoid fever. It is conceivable that 
such persons may finally contract the fever. In fact, Davies and 
Hall® report a carrier (Mrs. H.) who apparently caused three out- 
breaks of enteric fever and who finally had herself an irregular 
febrile diarrheal affection which the authors considered to be typhoid 
fever. If this diagnosis was correct it means that Mrs. H. had a 
period of incubation’’ of at least three years; that is, the typhoid 
bacilli were contained in her intestinal tract for a period of three 
years before she herself contracted the disease. 
Such occurrences may explain the development of the case at the 
Government Hospital for the Insane (see p. 35). 
The following table shows the progress of the disease as indicated 
by dates of onset of the cases in periods of a half month. 
The number of cases for corresponding periods in 1907 and in 1906 
are placed in parallel columns. 
April 1-15, incomplete 
April 1&-30, incomplete 
May 1-15, complete 
May 16-31, complete 
June 1-15, complete... 
June 16-30, complete 
July 1-15, complete 
July 16-31, complete 
August 1-15, complete 
August 16-31, complete 
September 1-15, complete.. 
September 16-30, complete. 
October 1-15, incomplete . . 
October 16-31, incomplete. 
Period. 
1938. 
1907 
1906. 
32 
32 
101 
131 
139 
70 
59 
41 
98 
22 
a Lancet, November 28, 1908, p. 1585. 
