The plain people long ago crystallized their experience in diet in the maxim that what is one man's meat is another man's poison, but your scientist will never believe anything until he sees it in a test-tube and physicians have kept on a few centuries behind the rest of the world prescribing _diet_ for all cases of the same disease irrespective of whether or not it agreed with the patient. Witness the rigid _diets_ for tuberculosis and Bright's disease. So, inevitably, there had to be a _diet_ for hay fever and equally inevitably, the same diet did not agree with everybody. Scientific men are fond of stating in scientific terms what everybody else knows already. While we have known for a long time that some foods did not agree with everybody, science is just now demonstrating that one man's meat is literally another man's poison by testing the different food proteins on the skin and calling the condition _food anaphylaxis_ or _food allergie_. As the patient reader of the chapter on Pollens will remember, the anaphylaxis or sensitiveness of the patient to particular pollens is tested by rubbing a speck of different pollens into scratches on the skin. This skin reaction as a test of anaphylaxis was used by Schloss with different foods before it was adopted in hay fever; and it has been taken up by the dermatologist also. The dermatologist has long suspected that certain skin diseases, as urticaria, and eczema, are aggravated or produced by certain foods but he has been unable to demonstrate just what foods were at fault. The problem was confused by the fact that he had found no guiding principle. Food that one patient could eat with impunity brought out a beautiful eczema or urticaria on another patient. The uric acid theory was one effort to solve this problem but it was not comprehensive enough and it was not true. Forbidding _nitrogenous foods_ has been a favorite formula with some and they straightway advised milk, which is highly nitrogenous. The recent recognition that food sensitiveness is an anaphylaxis and the detection of the foods at fault by the skin reaction may supply the missing guiding principle that was needed to adjust a diet to the individual needs. The poisonous element in food is the protein. The food itself will serve for the skin test but it is better to use the pure food protein, which gives clearer reactions and avoids contamination. Proteins of all our common foods are now obtainable in the drug trade put up in tiny capsules ready for the test.