Flu season was a time of year in which one was more susceptible to the flu.
Neelix was convinced he had the Toluncan ague every flu season. In 2376, following The Doctor's (brief)resignation and prior to his departure from USS Voyager, he reminded his medical assistant, Tom Paris, of this fact about Neelix and instructed him to give the Talaxian a placebo when the time arose. (VOY: "Virtuoso")