Although variable weather is hard on them is doesn't cause illness, per se. My birds are dealing with up and down temps this year and a lack of turnout as are many, I'm sure. They don't end up raspy. (In fact, I believe the heat of summer is MUCH harder on the birds than the variable conditions of winter. Closed coop conditions in winter are far more likely to blame than temperature.) Respiratory "bugs" are always ALL around us and stressed birds are probably far more likely to succumb. I am one that will treat if there is a reasonable expectation that I will end up with a healthy/viable/productive bird at the other end but I also want a naturally strong flock. So I do my best to keep the birds in fresh air and relatively dry, well fed and with clean, unfrozen water. And even in my short time with chickens, I have learned to cull sooner rather than later if it looks like someone is really going "south".