Why is a definite article used before flu?

Answer 1

"The flu" is a familiar, almost slang way of saying influenza. Such familiar or shortened terms often take the definite article. Thus we suffer from depression or "the blues," from delerium tremens or "the D.T.'s," diarrhea or "the runs."

Answer 2

The definite article "the" is used before "flu" to show that we are specifically referring to a particular instance or type of influenza. It indicates that the speaker is talking about a known and specific form of the illness.