To answer this question, let's go back to 18th-century Italy ...
The calendar may say that spring has sprung, but until recently some parts of the country were still experiencing unseasonably chilly weather. As we’re all well aware, the brutal cold from last winter came with a nasty flu season, too. But while we’re finally past the peak of the flu season, some areas are still reporting cases of Influenza B (A was the predominant strain for most of this past season). So, yes, it’s still possible to catch the flu in May … even into June in some areas.
In fact, the “end” of the flu season is never announced by the CDC; the virus is always active — albeit to a much lesser extent once the warmer weather hits. So why exactly to we get hammered by colds and flus during the chilly winter and early spring months, and get some respite from them during the warmer months of the year?
The answer might lie in the origins of the name of “the flu” itself: some historians believe that the modern-day word for the virus, “influenza,” actually comes from the 18th-century Italian phrase for the virus, “influenza di freddo,” which means “influence of the cold.”
The Italians were likely on to something, as one of the leading scientific theories behind the flu season’s occurrence during the winter months is that the flu virus is at its most stable in the cold, dry air that is so pervasive from November to March (in the northern latitudes) and from May to September (in the southern latitudes). Another popular theory is that during the winter months, people are more likely to be crowded together indoors, which leads to greater transmission of viruses (especially aerosolized viruses like flus and colds) between individuals – although this theory is more contested than the cold-weather theory.
Whatever the reason, the warmer springtime weather is good for more than just lifting your spirits and breaking a bad case of cabin fever: it’s actually a good sign that the flu season is over or will soon be ending.
Of course, it’s still an excellent idea to practice good flu- and cold-prevention techniques year-round. Although less common, cases of the flu and colds do absolutely occur outside of the winter months – and often, people report feeling sicker in the summertime if they do catch a bug (but this may be more of a psychological phenomenon than any real increase in symptom severity).
So just because you’ve abandoned your snow boots in favor of flip-flops and your wool sweaters in favor of tank-tops doesn’t mean you should abandon the good health and hygiene practices that (hopefully) kept you flu-free during the winter time. Unlike the flu, hand-washing, eating right, exercising moderately, and getting enough sleep know no season.