Some testing can be done on live birds, for certain things.
Some times it takes a necropsy on a recently dead bird to find an answer.
Practicing strict biosecurity and not bringing home random birds goes a long way to prevent disease entering your flock.
Try this.
All state lab's are listed here:
https://www.backyardchickens.com/threads/necropsy-and-disease-testing-lab-info.1236884/
Click on the blue Link above.
Uh oh. Could you not see the link I posted?
Maybe it's not showing up since I copied it.
Here can you see this one?
http://cahfs.ucdavis.edu/submitting/index.cfm
California's state lab.
If you want to find out why...you need to send a body here:
I believe it only cost $25.
Well worth the money to find out if you have a disease spreading through your flock.
It might be easily fixable with certain meds.
California
http://cahfs.ucdavis.edu/submitting/index.cfm