It appears that conditions of destitution, filth, and intemperance amongst an overcrowded population favor the development of the virus, and hence the epidemics have, as a rule, begun in towns, such as Dublin, Glasgow, Odessa, St. Petersburg, Breslau, etc., where such conditions prevail. Great importance has been attached, in particular, to the scarcity of food and to destitution as powerful factors in favoring the production of the disease. Some of its names (hunger-pest, hunger-typhus, famine fever) have been given with reference to this, and in the case of several outbreaks a careful comparison has been made of the decrease of the food-supply and the consequent advance in price of the staple commodities with the development and progress of the disease. Although this is in all probability true of those centres where relapsing fever originates, it has but a partial application to the secondary centres where the disease is imported and develops.
The presence of destitution and filth, enfeebling the vitality of a section of the community, would favor the spread of this as of any other specific fever, but there is considerable evidence to favor the view that the importance of starvation as a cause of the fever has been exaggerated. This was strongly urged by Parry5 as the result of his study of the Philadelphia epidemic of 1870, and our own more extended observation showed that the vast majority of the patients appeared to be well fed. On the other hand, the influence of overcrowding as favoring the development and spread of relapsing fever has been clearly established by the study of many epidemics, as in the Breslau attack of 1868, reported by Wyss and Bock, where single tenement-houses furnished as many as seventy-one cases; in the Edinburgh epidemic of 1869 and 1870, where Muirhead found the breathing-space allotted to each individual in the affected houses to vary from 250 to 400 cubic feet; and in the Philadelphia epidemic, where the observations of Parry and ourselves showed the presence of an extreme degree of overcrowding in most of the houses where the disease broke out.
5 Loc. cit., p. 339.
No age is exempt, but neither can it be said that age exerts any influence upon the occurrence or frequency of relapsing fever. Of 1164 cases in the Philadelphia epidemic of 1869-70 in which the age was noted, the result was as follows:
| Males. | Females. | |
| Under 20 | 149 | 76 |
| From 20 to 30 | 220 | 140 |
| From 30 to 40 | 143 | 101 |
| From 40 to 50 | 135 | 67 |
| From 50 to 60 | 60 | 34 |
| From 60 to 70 | 20 | 6 |
| From 70 to 90 | 6 | 7 |
| Total | 733 | 431 = 1164 |
The youngest cases were in children two or three years old; the oldest patients were women over eighty-five years old.
Sex exerts no influence, though, on account of the larger proportion of males likely to be exposed to the specific cause, the results of nearly all epidemics show a preponderance of male patients in the proportion of 33 per cent., 66 per cent., or even 85 per cent. (Meschede).
Nationality does not act as a predisposing cause,6 except in so far as certain countries may present more frequently than others the conditions favorable for the development of this disease. Of 1170 cases in Philadelphia in which the nativity was noted, 219 were Irish, 61 English, 161 German, 729 American. Of the latter 729, about one-half, or nearly 28 per cent. of the whole number, were negroes, while the negro population of Philadelphia was only about 3.3 per cent. of the total. This excessive proportion of cases among the negroes was undoubtedly due in large part to the fact that in Philadelphia overcrowding is notoriously more common and extreme among them than in any other portion of the population, although it is also likely that they present an excessive susceptibility to the virus of this as of many other specific diseases.
6 Hirsch's Geog. and Hist. Pathology, New Syd. Soc. ed., 1883, vol. i. p. 615.
Attempts have been made to show some connection between the period of the year or the atmospheric conditions and the rise and spread of epidemics of relapsing fever; but, as Murchison clearly showed, these epidemics are wholly independent of such influences. In Philadelphia, of 1176 cases in which the date of occurrence is known, there occurred in September, 1869, 4 cases; December, 1869, 6 cases; January, 1870, 5 cases; February, 1870, 13 cases; March, 1870, 124 cases; April, 1870, 209 cases; May, 325 cases; June, 293 cases; July, 115 cases; August, 19 cases; September, 28 cases; October, 15 cases; November, 1 case; December, 2 cases; January, 1881, 2 cases; February, 1 case; March, 2 cases; May, 7 cases; June, 2 cases; September, 2 cases; October, 2 cases.