This is a classic symptom of Marek's Disease. Much information about it on the internet. It's highly contagious. There is a vaccine. Some birds display no symptoms, yet carry the disease and will pass it to others through inhaled dander shed in the environment.
Any of your chickens that survive this outbreak, may be Silent Carriers for the rest of their lives. The only solution for replacing chickens is to purchase vaccinated stock, which some hatcheries do sell.
You should thoroughly clean and disinfect the coop and as much of the yard as you can and remove/quarantine any chickens displaying symptoms.
You can vaccinate your own chickens. You'd have to study up on how efficient it might be for chickens already exposed & Silent Carriers.
The disease poses no threat to humans. Selling chickens from your flock would be unethical and would likely spread the disease to other flocks.
It might be possible that some toxin, not disease, has been ingested by your chickens. When things like this rise up in a flock, it's good to remove and watch individuals to see if any other symptoms might arise. That's the best way to diagnose at home.
Lab work at the vet's office would likely be very expensive, but could more accurately determine the cause of this paralysis and death.
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