Health and Disease 
system, and yet it is of the utmost value to postulate one that 
is frictionless in order to isolate other elements of it. 
The complete description of what would be wrong with a 
germ-free world, how to go about achieving it, how it could be 
maintained and what the imbalances are that might flow from 
it, might be much more intersting and provocative than any of 
the partial steps towards it, and perhaps we ought to discuss 
it in more detail. What fundamental basic limitations would 
there be in maintaining a germ-free world and what would be 
its danger? The implication is that it would be dangerous 
because we would be extremely vulnerable to the introduction 
of a single micro-organism which otherwise might not be patho- 
genic. Why is it pathogenic under those circumstances? Is it 
only necessary to maintain some reasonable level of activity 
of the reticuloendothelial system in order to have a ready res- 
ponse? In this case you could just take a shot of a mixed load 
of antigens every few weeks to keep yourself pepped up pro- 
perly and you wouldn’t have to worry too much about the next 
micro-organism. It is only by pushing these abstractions to the 
limit that we are going to be irritated into thinking about 
questions that are a little bit more general than the immediate 
ones of today. 
Koprowski: Dr. Medawar answered his own question about 
polymorphism and resistance by stressing the role of the variant 
forms of large molecules in determining different responses of 
human subjects to exposure to an infective agent. The different 
permutations and combinations of genetic material which 
resulted in what Dr. Medawar calls chemical polymorphism of 
the human population have undoubtedly made it impossible for 
one pathogen to annihilate the human race. Although resis- 
tance of man to infections is conditioned by the genetic diversity 
of his molecular constituents, the actual modus operandi of these 
factors is unknown and should be thoroughly investigated in 
man himself. Studies of experimental animals were conducted. 
as I have pointed out in my talk, in genetically uniform breeds, 
If the mechanism of resistance of man to a large variety of 
pathogens were to be unravelled, it might become possible to 
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