Treating what seems to be a recurring respiratory illness - possibly environmental?

[Lyris]

I've long since accepted that I have what appears to be an eventually fatal respiratory illness within my flock. Sneezing, coughing, congested breathing... It's responsible for the deaths of four of my chickens and seems to be antibiotic resistant. A couple of my chickens ended up with another home (they're aware of the illness and my chickens are the only ones they have) I've kept in contact with them and it appears they've had much better luck than I have. Aside from some minor sneezing during times of stress, they haven't had a single problem. Mine never fully get rid of it and it just gets worse. I'm wondering if it's possible that there's something in my environment that's possibly making it worse.
My chickens sleep in an old wooden shed I've repurposed for them. The shed itself is big enough that I could lay down in it comfortably. It's as tall as my house and has actual windows. Despite the many different types of perches and roosts I've given them they all prefer to sleep on the built in bookshelves so I have mini perches on various shelves for them. is it possible the old wood could be making their issues worse? Maybe dust? Could I put something over the wood (straw, sand?) to help keep down dust or any illnesses? I can't make major renovations to it since 1) I'm incredibly poor right now and 2) It's actually a structural part of my carport.

I'm not sure if this should go into illnesses or coop design, so sorry if I get it wrong.