My mother has always told me that the flu seemed to be worse when she was growing up in California (now, she lives in Montana.) I can vouch for the effectiveness of the little flu germs down here. They seem to be a lot more potent; a few years ago, a flu kept me bedridden for over five days. That was horrible. Today, I saw a little anecdote while reading a Science Blog article about having sex in the summer:
"A study released last fall by Oregon State University found that summer is a bad time for bacterial infections in general, with serious infections caused by bacteria up as much as 17 percent with every 10-degree increase in seasonal air temperature."
I'm willing to bet that a similar statistic applies to geographical temperatures, as well.
Incidentally, the rest of the article seemed like common-sense stuff and wasn't particularly noteworthy; e.g., steeping in the hot tub bad for fertility, E. coli lives in water and is bad, etc, etc.