***influenza virus really does spread more effectively in cold, dry air***
Yet the 1918 flu also hit the Eskimos and the tribes of the hot, humid Amazon basin hard.
It would seem to me that if there are enough viri they could overwhelm any deleterious-to-them environment, and that was likely the case with the Spanish flu. Sanitation in the first world, San Francisco notwithstanding, has greatly increased in the last 100 years.