THE MICROBES OF HUMAN DISEASES. 215 



active principle of the vaccine virus. The liquid need 

 only be deprived by filtration of its micrococci in 

 order to become inert, and consequently unfit for use 

 in vaccination. 



X. THE MICROBES OP CROUP AND WHOOPING-COUGH. 



The parasitic nature of croup and diphtheria, which 

 had long been suspected, was only shown in 1881 

 by the researches of two American physicians, Wood 

 and Formad. In the spring of that year a very 

 serious epidemic of croup occurred at Ludington, a 

 small town on the borders of Lake Michigan. Here 

 the principal industry is derived from the neighbouring 

 forests, the trees of which are sawn into planks in the 

 numerous saw-pits, and thus employ almost the whole 

 working population. The town stands on a height, 

 with the exception of one quarter of it, which is built 

 on very low, marshy ground, partly filled up with saw- 

 dust. Here the soil is so wet that when a small hole 

 is dug, it fills with water immediately, and cellars are 

 almost unknown. It was in this quarter that the 

 epidemic was so severe ; almost all the children were 

 attacked by it, and a third of them had already died. 



Formad went to Ludington to study the epidemic 

 and collect materials for experiments. In all these 

 cases of croup, the blood was full of micrococci belong- 

 ing to Micrococcus diphthericits, some detached, others 



