FUNGUSES 59 
favourable. Exactly what the favourable conditions are 
is not known, nor why, with the people exposed to the 
same infection, one should be seized on by the bacteria 
as a suitable feeding-ground and the other go scot-free ; 
but the object of vaccination and similar operations is 
to put some substance into the blood which will make 
it unsuitable for the invading fungus, and never let it 
multiply. Even when it has taken a hold of the body 
substances may be injected into the blood which are 
thought to be fatal to the growth of the guest, as is 
often done now when the diphtheria fungus has taken 
possession of a patient. 
When the bacteria have absorbed all the nourishment 
that suits them they may either die or may form spores 
from their cells. These spores, forming bacteria, are 
very dangerous, for they take a great deal of killing, and 
can wait for a long time without food in this condition, 
and are very easily blown about to spread infection. 
Very little higher in the scale are the yeast plants, 
which secure the rising of bread and the fermenting of 
wine and beer. The actual effect of their growth is to 
split up the sugar in their hosts into two substances, 
alcohol and carbonic acid. When the yeast plant is well 
kneaded up with flour and water the bubbles of gas 
are caught in the tough dough, and the whole mass rises. 
Then, in the baking, the alcohol evaporates, and the 
bubbles of carbonic acid swell up with the heat, and 
make the loaf fit to eat. When beer is made, on the 
other hand, the carbonic acid is mostly allowed to escape, 
and the alcohol to accumulate until the required strength 
is reached. Guinger-beer is made in much the same way, 
except that the yeast plant is only allowed to grow until 
it. has made a very small amount of alcohol, and the 
