Disease the entomologist caught? (3)

I believe the answer is:

bug

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'disease' is the definition.
(thesaurus)

I don't understand how the remainder of the clue works.

(Other definitions for bug that I've seen before include "Insect, or flaw in computer program" , "Tap" , "Insect or defect in a computer program" , "Software fault" , "Irritate" .)

I've seen this clue in the Evening Standard.

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