The worst of flu season is November through February (at least in Boston) although it can be shorter or longer; flu should be considered in anyone presenting with a fever and cough or other viral symptoms during this time of year.
Flu is a clinical diagnosis (see below) which means patients do NOT need a positive test result to get treated! If you think someone meets criteria for treatment, you can empirically give them antiviral medication. Plus, getting tested for flu is no fun: it’s a nasal swab, which has to go all the way back into the throat. However, as described below, certain kinds of patients should be tested.