Some of it may actually be an illness non-chill-related. Could even be vaccine related in a few cases. The ones who drove their chicks home in a warm car have the best case for it, definitely.
But, for the others, I'd like to point out again that chilling CAN actually cause problems that drag on for weeks and cause the bird to succumb as it grows: fluid in the lungs or body cavity, poor digestion, lack of immune system development (I believe this is specifically related to the organ that produces B cells, the bursa, but there may be other factors), slowed growth rate, poor heart development and leg development, specifically. This information is widely available. Some hatcheries used to mention some of it on their websites, for instance. Apparently, McMurray might do well to include it too.
Here's an article that discusses a bit of this:
http://www.poultryscience.uark.edu/pdfs/avian_advice_5.1.pdf
People also forget how individual a process shipping is: trucks can be warm or cold, wait times between destinations vary, handling varies, etc. No experience is going to be like any other. But one thing is certain: the earlier in the year you order baby poultry, the more likely you'll be to have problems.
What are the odds that the oldest hatchery in the country (that also happens to be one of the most northerly ones too) is selling chicks with an X-Files virus? In February and March? No one could say for certain, of course, because strange things do happen. And I don't even claim that it's all related to one cause. But I do think that it's
mostly related to the most simple and obvious cause, which does not always produce simple and obvious effects. And I'm willing to bet that as spring gets nearer, there'll be fewer of these stories.
I hope so anyway. My genuine sympathy to those who've had problems.