Mold is the only other thing that causes paralysis in young chickens.
Avian viruses start producing symptoms between eight weeks and five months.
There is always the truly freaky. Years ago, I had several baby chicks find a buck moth caterpillar and they pecked it. It had a powerful venom...
You are correct that these chicks are too young for this to be from Marek's.
I've had experience losing chicks to the symptoms you describe. One day, I was sitting in the run when an eight-week old chick came in and collapsed at my feet, literally its legs going out from under it. The chick...