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English definition of “bug”

bug

noun [C] (ILLNESS)    /bʌɡ/ informal
B2 a bacteria or a virus causing an illness that is usually not serious : I had a tummy / stomach bug last week . There's a bug going around (= an illness that many people are getting).Bacteria, moulds, germs and viruses
(Definition of bug noun (ILLNESS) from the Cambridge Advanced Learner's Dictionary & Thesaurus © Cambridge University Press)
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