Jill Diskin had heard the story. So had Amelia Martin.
Both of them grew up hearing their parents warn them about catching colds when the weather suddenly turned warm and wet in the winter, just like this weekend, when highs were 20 degrees above earlier in the week.
"I was just thinking about that looking at these people out here playing Frisbee and nobody has their legs or heads covered," said Ms. Diskin of Squirrel Hill, who was walking under an umbrella around a Schenley Park track. "When I was growing up, you wore a scarf and you wore leggings and boots all the time. A warm snap was called pneumonia weather."
Ms. Martin, a Dormont resident who was one of the Ultimate Frisbee players, believed the same thing. "My parents did hint at that. You had to wear your big coat and don't get wet. I think it's probably true. I don't know if it causes the body's defenses to go up and down, or what."
It's even enshrined in one of our most iconic holiday films, "It's a Wonderful Life." When Zuzu catches cold walking home with her coat open, a distraught George Bailey shouts at her teacher: "Say, what kind of a teacher are you anyway? What do you mean sending her home like that, half naked? Do you realize she'll probably end up with pneumonia on account of you?"
With all due respect to Frank Capra and Ms. Diskin's and Ms. Martin's parents, there is very little scientific evidence that sudden temperature changes make people more vulnerable to colds or the flu.
Sheldon Cohen, a Carnegie Mellon University professor who has done experiments showing the link between stress and getting colds, said the peak of flu season is usually right after children come back from Christmas vacation -- meaning that crowding people together where the germs can spread is the major reason for epidemics.
The other driving force is the simple fact that flu and cold viruses become virulent in the winter in both hemispheres of the globe (meaning flu season in Australia is in the middle of our summer), for reasons that aren't fully understood.
Andrew Nowalk, an infectious disease expert at Children's Hospital of Pittsburgh of UPMC, said one reason people may associate temperature changes with illness is that it's easier to remember getting sick after an unusual weather shift than the rest of the time.
"When we have a cold winter that stays cold and we have a high rate of flu, nobody comments on the fact that the winter is cold, but if you have a winter with the same rate of influenza with a few 65-degree days thrown in, people will all be talking about the change in the weather. It sticks in your mind so it's kind of a mental place marker."
There is some research on the relationship of temperature and respiratory illness, and most of it points to colder, drier conditions being the primary risk.
Ronald Eccles, director of the Common Cold Centre at Cardiff University in Wales, hypothesized in a 2002 paper that colder conditions cause blood vessels to constrict in the lining of the nose, leading to "inhibition of local respiratory defenses" by making it harder for defensive blood cells to get to that site. "The inhibition of respiratory defenses is sufficient to convert a subclinical infection to a clinical infection," he wrote.
And Peter Palese, a researcher at Mount Sinai Hospital in New York, found in experiments with guinea pigs that cooler, drier temperatures were most likely to allow one animal to infect another with the flu. The drier air probably allows the virus to stay in droplets in the air for longer, he said, and it also may break down the tissue lining the breathing passages.
On the other hand, he said, being outdoors would actually decrease chances of someone getting the flu. "Unless they are five inches away from each other coughing into each other's faces, being outside, running around outdoors, that is not as conducive to the transmission."
While temperature and humidity may be minor factors in spreading colds and flu, they are overwhelmed by the fact that these viruses are more potent in the winter at the very time we all tend to be crowded together in indoor spaces more often, Dr. Nowalk said.
It's why getting preventive flu shots is so important.
"If you look at the statistics of mortality from influenza, if you can make a 1 percent reduction, it makes a huge difference in the numbers who die. That's why flu shots are so important, and it's by no means too late for people to benefit from them."
Noting that grandmothers the world round may have warned children about temperature spikes and the flu, he said:
"I understand grandma's desire to blame illness on the temperature, but I sure hope grandma got a flu shot before she handed out that advice."
Mark Roth: firstname.lastname@example.org, 412-263-1130 or on Twitter @markomar.