666 PROCEEDINGS OF SECTION H. 
has been no satisfactory proof that the disease can be produced in 
animals, either by the bacilli or their spores, whether they have 
been introduced into the stomach with food, inoculated below the 
skin, or injected into the blood. Till this link in the chain of 
evidence has been supplied, the proof of a causal relation between 
the organisms and the disease can hardly be looked on as perfect. 
Assuming that the Bacillus typhosus, of Eberth and Gaffky, is the 
true infecting agent, it is of importance to take note of its vital 
properties. It grows freely on suitable media at ordinary room 
temperatures, but has not been found to develop spores at a 
lower temperature than 68 deg. F. (20 deg. C.) Spore formation 
goes on most actively between 86 deg. and 104 deg. F. (30 deg.— 
40 deg. C.), and it ceases at temperatures over 107-6 deg. F. (42 . 
deg. C.) Organisms which grow in diverse media, and at such 
varying temperatures, evidently possess a high degree of vitality. 
The spores, especially, have great power of resistance, and they 
have been kept in the dried condition for more than three 
months, and then found to germinate freely. The most recent 
researches, if they do not directly confirm, certainly do not con- 
tradict the view generally held by sanitary authorities, that the 
specific virus of typhoid grows and multiplies outside of the body, 
in drains and cesspits, and possibly also in the soil and in water. 
Whatever may be the case with the fully-developed bacilli, it may 
be taken as certain that the spores will survive for a considerable 
time, either in the moist or dry condition, and that on suitable 
media they will germinate and multiply rapidly at ordinary 
summer temperatures. That the spread of the disease is actually 
due, in large measure, to this multiplication of the virus outside 
of the body, and its conveyance in some way to susceptible per- 
sons, is further confirmed by the following facts. It is possible 
that the disease may spread directly from person to person, but 
all medical authorities are agreed that this is not the ordinary 
mode of infection ; the contrast, in this respect, between typhoid 
and such diseases as measles and small-pox being very marked. 
And further, the circumstance that typhoid is, in almost all 
countries, a disease most prevalent in the late summer and autumn, 
strongly suggests some affinity between it and the miasmatic 
diseases, like the malarial fevers. And if anything further were 
wanted to establish this, it would be found in the undoubted fact 
that the prevalence of typhoid in towns or districts is very 
largely dependent on their sanitary condition, accumulations of 
filth and defective drainage favouring its spread, and improve- 
ments in these respects tending greatly to keep it in check. 
There is another point in connection with this question of 
varying prevalence which is of importance. All epidemic diseases 
have successive periods of greater and less degrees of severity, 
both as regards prevalence and fatality. In the case of purely 
contagious zymotics, like measles, the recurrence of severe out- 
