If you have seen the classic one leg forward, one leg back, that is likely Marek's. Although, there are some other illnesses which mimic Marek's such as Botulism. With your high heat, you could be growing some manner of pathogen in the coop/run.
BYC has a very good, imho, definitive article on Marek's here:
https://www.backyardchickens.com/a/the-great-big-giant-mareks-disease-faq
On the other hand, Marek's doesn't tend to spread that quickly and cause overnight collapse. It tends to be a bit slower, lame and limping one day, slowly growing worse, still eating/drinking, then wasting until death or recovery...slowly picking off your flock. Technically, if you believe the statistics, all chickens are Mareks carriers as Mareks is prevelent. It is like having the herpes virus in your body and only producing the cold sore when your immune system is down, which stress can do. Raising chickens with Turkeys can create an immunity to Marek's as Turkeys usually carry the Turkey Mareks which is much, much milder in chickens and often gives immunity to Chicken Mareks (original vaccines were based off the turkey virus)....akin to cow pox protecting humans against small pox. But Mareks is morphing into even more strains such that the industry is having a hard time keeping up with it. And, yes, there is even a super virus now that I think about it, that does move quickly. Hopefully you haven't stumbled onto that! (Let's assume not as that is not prevalent yet).
But you've got birds of all ages affected, quickly. That could be coccidiosis, which can very quickly emmaciate and weaken, and cause collapse overnight (but not one leg forward/one leg back paralysis). Coccidiosis can be brought on by stress as it too is something that is never eradicated from the bird's system, only held in check by a healthy immune system. Coccidiosis does not have to produce bloody poo to be an issue. It depends on where the protozoa (coccidia) have taken up residence whether or not it gets into the lower track to produce bloody poo.
If they were my birds, I would treat with Sulmet (a sulphur drug) rather than Corid. It is cheaper and it would give you antibiotics against the common gram-negative bacteria in case it is something that has taken hold in the heat. Follow the directions on the bottle treating full dose 2 days, then half dose 4 days. You often see great improvement by day 2.
And perhaps post a photo of the bird with the leg position. It could be classic Marek's working its way through. Support with ACV, yogurt, Chick Saver in water to boost immune as much as you can and hope for the best. Some birds will succumb to the tumors while others will keep them at bay. It's the way of the disease. Breed from those that survive and thrive as they will have a natural resistance.
I agree the only way to know for certain is to send/take the bird in for a necropsy.
My 2 cents.
LofMc
