Imagine the first sentence in a news report:
Scientists have discovered the evolutionary causes of allergy.
Could we write:
Scientists have discovered the evolutionary causes of the allergy.
According to the Longman Dictionary, allergy can be used as a count noun as well as a noncount noun, so by adding the we are creating a generic noun phrase, similar to this:
Scientists have discovered the evolutionary origins of the dog.
However, I feel that the allergy is not a good choice.
Maybe diseases are too "personalized" to fit in "definite generic" constructions?
P.S. I suspect that allergies might be wrong too. Am I right? Well, not that I feel it personally; I just remember a passage from Quirk et al.'s "Comprehensive Grammar" (Unit 5.52) where the plural noun phrase in "Nora has been studying medieval mystery plays" is indicated as not truly generic. I wonder if the plural might be okay here. The more I think about it, the more I get confused.