GROWTH OF EPIDEMICS. 15 



washed with water contaminated with sewage ; or, perhaps, a little 

 of that element has been added to improve its quality ! then fol- 

 lows what is significantly called " the trail of the milk-man " a 

 trail marked by fever cases in perhaps every house to which the 

 milk has been distributed. 



Or there is a fever at one of our town dairies, in the back-room 

 of the shop, for example, and the attendant on the sick also 

 attends to the customers, who carry the milk to their homes, and 

 with it the germs of future disease. This diagram will assist you 

 to realise more vividly what I have just said. It represents a 

 farm-house in which typhoid fever has arisen, by the milk becom- 

 ing tainted in the way I have spoken of. This dot marks the 

 first case which arose in consequence. 



Observe how the disease spreads from this starting-point. This 

 other dot shows the second case, which arose in nineteen days 

 from the first, and this, the third, in twenty-six days from the first 

 all in the same house; but in fourteen days from the first case, 

 a group of three cases occurred in a family, supplied with milk 

 from this farm, and within eight weeks from the first case, a hun- 

 dred and sixty-six persons were laid down with the fever. 



Look now at this diagram, illustrating the growth of an epi- 

 demic of scarlet fever, from a single case. 



It is a moderate estimate of the rate of progression of this fever, 

 to say that each case produces two others. Follow these dots 

 from the starting case, and in seven weeks from its commence- 

 ment, you will see that the one case has multiplied into two hun- 

 dred and fifty-six. Once more, study with me this diagram 

 marked cholera. I have said that this disease is propagated by 

 water, fouled by choleraic discharges. Estimating that each case 

 produces five others, a ratio which may be taken as considerably 

 under its usual rate of progression, there would arise six hundred 

 and twenty-five cases within eight days. Several instances 

 of this actually occurred during the cholera epidemic of 1866. 



The presence of an epidemic of small-pox in London, and 

 the certainty of its early advent amongst ourselves, lead me to 



