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 Microses or 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 diphthericus, some detached, others 
