HOW DO MICRO-ORGANISMS CAUSE DISEASE ? 
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eluding some of the most important and cor 1 ' imon that affect 
man and animals, was absolutely unknQwri to us ; that to-day 
we possess very definite and accurate information on these 
points in the majority of them ; that month by month, and 
almost day by day. Our knowledge concerning them becomes 
more and moc extensive, more and more precise, we shall, 
perhaps, appreciate a little more correctly the immense ad¬ 
vances that scientific medicine has made in the last three 
decenniums. 
Starting from the broad and firm basis of experimental 
physiology and pathological anatomy, we may to-day divide 
all known maladies into the following four groups: 
1. Diseases of traumatic origin. 
2. Diseases of infection. 
3. Diseases due to nutritive changes. 
4. Neuroses. 
Of course there is no essential difference between the ori¬ 
gin of classes one and two. Micro-organic infection is a trau¬ 
matism of the cells. But it is convenient to separate those 
affections that are due to rough macroscopic influences from 
those due to the less visible and secret injuries inflicted 
by the microbes on the cells. Classes three and four are 
those about whose rationale we know least; but they are also 
those that suffer most from the discoveries that are made. 
The number of neuroses and nutritive diseases diminishes 
continually, as disease after disease is found to belong among 
the infectious. As far as the neuroses are concerned, it is 
well understood that the class is only a provisional one. The 
change may be so minute as to elude our grasp for a long 
time to come, but we may venture to affirm that there prob¬ 
ably is an actual change of tissue in every case, and when 
there is such a change there must also be a cause for it. 
Three names stand pre-eminent in the history of our 
knowledge of the infectious diseases. The first is that of the 
man who discovered the specific microbes that cause the fer¬ 
mentations, alcoholic, acetic, lactic, by means of which alone 
the life of the higher animals becomes a possibility.— Pasteur. 
The second is that of Lister , who adapted and utilized these 
