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. 
Peeiod of 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, state 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 ma}" be noted that: 
1904., case 2 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; 
he showed first symptoms 7 days after first tick bite. 
1904, Gates’s case 16 showed symptoms 5 days after tick bite. 
Not arguing on the tick hypothesis, it may be noted that — 
1904, case 3 moved in January to house in which she was taken sick on ]\Iay 3, 
making a maximum of about 4 months in the infected locality before she became ill. 
1904, case 10 left Iowa 6 weeks prior to onset, 5 of these 6 weeks being spent in 
Bozeman, Mont. ; thus this patient was in Montana a maximum of 6 weeks prior 
to illness. 
1904, case 11 had been in the United States 4 weeks, in the Bitter Boot Valley 19 
days, and at the house where she was taken ill 14 days prior to onset of the 
disease. 
1904, Gates’s case 15 was taken ill about 7 days after he visited the locality in which 
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 
stage. 
These data would indicate that either upon basis of the tick hy- 
pothesis or upon basis of infection by some other unknown method, 
the disease may develop within 6 days after exposure; the maximum 
