40 
the age and sex and general health of the patient appear to have no part in deter- 
mining susceptibility to the disease; a large number of cases give a history of recent 
exposure to wet or cold or of overexertion shortly before the attack, but in several 
cases all such history has been absolutely excluded; many of the patients have suf- 
fered somewhat from indigestion and constipation immediately prior to the attack, 
in others no such condition existed. 
Period op Incubation. 
Idaho . — The period of incubation is uncertain, probably 10 to 21 days (Bowers, 
1896, p. 63) . 
Montana . — Gwinn (1902) says that in so far as exposure to wet and cold may be a 
cause, the onset is on an average of 18 hours after exposure. Anderson (1903a, p. 
21) gives the incubation as 3 to 10 days, usually about 7. Wilson and Chowning 
(1904a, p. 37), arguing on the theory of transmission by ticks, sta^ that though all 
cases occurring in 1902 and 1903 gave a history of tick bites (see, however, p. 30) 
shortly before the onset of the symptoms, only the following cases gave a clear history 
of a definite incubation period : 
Days. 
Cases 94, 97, 117, 124, 125 2 
Case 96 3 
Case 119 5 
Case 116 6 
Case 112 7 
Case 115 8 
Cases 120, 121 2 to 5 
Several of the cases which occurred in 1904 give points of com- 
parison upon the period of incubation. 
Arguing on the tick hypothesis, it may be. noted that: 
1904, case ^ was bitten by ticks April 24, and fever was noticed the following day. | 
1904, case 8 was bitten by ticks May 15, and was taken sick the following day. | 
1904, Gates’s case 15 started for Clarke Fork Canyon May 29, remaining in that [ 
vicinity about 4 days, during which time he was bitten by ticks in five or six places; j 
he showed first symptoms 7 days after first tick bite. ■ 
1904, Gates’s case 16 showed symptoms 5 days after tick bite. j 
Not arguing on the tick h}^pothesis, it ma}^ be noted that — j 
1904, case 3 moved in January to house in which she was taken sick on May 3, i 
making a maximum of about 4 months in the infected locality before she became ill. i 
1904, case 10 left Iowa 6 weeks prior to onset, 5 of these 6 weeks being spent in i 
Bozeman, Mont. ; thus this patient was in Montana a maximum of 6 weeks prior j 
to illness. 1 
1904, case 11 had been in the United States 4 weeks, in the Bitter Root Valley 19 ;i 
days, and at the house where she was taken ill 14 days prior to onset of the | 
disease. : 
1904, Gates’s case 16 was taken ill about 7 days after he visited the locality in which j 
infection is supposed to have taken place; and ■, 
1904, Gates’s case 16 was taken ill 6 days after sleeping with a case (15) in its initial jj 
stage. : 
These data would indicate that either upon basis of the tick hy- I 
pothesis or upon basis of infection by some other unknown method, 
the disease may develop within 6 days after exposure; the maximum jj 
