Flu season isn't the only season.

Its worth noting that when we talk about flu season we are talking about that time of year when flu cases peak. Its not flu season in the US now but there are still cases of influenzavirus spreading among humans. Its just not at epidemic levels (case numbers significantly above the norm).We are entering normal flu season in other parts of the world though, like down here in Aus, although cases are currently sparse. Cases of seasonal flu in one country can be easily spread to other countries by global travel. This is not just the case for influenza viruses of course. Any human respiratory virus ticks over at baseline levels outside of its main "season". Most of the time these infection come and go quickly and with relatively little illness.
Remember, viruses don't know the temperature outside, or the cloudiness - it's the hosts of infection that play the biggest role in virus activity.