You need to take pictures of the problem and the poop.
Most importantly seperate the sick one from the others so it doesnt spread.
It sounds like an uppoer respirotory infection that hit the sinuses (connected to eye duct) or a different kind of bacterial / viral infection.
They could have picked it up at your place or maybe they got it at the point of sale or before.
My guess is its something in your area from wild birds or latent in the wood if its an old barn.
Good news is it should clear up with treatment, but chicken are very succeptable to respiratory and digestive ailemnts so you need them to have clean areas to scratch (clean up the poo or have a good litter program) and great ventilation.
Get some pictures up so people can try to help, find out what it is and then treat accordingly.