MG is a fairly common in backyard flocks. It may not be a problem until there is some stress, such as during cold winter weather or during a molt, and then symptoms occur. Your other chickens have probably already been exposed the same time this one did. Most people close their flocks to new birds when a respiratory disease occurs. If you did a necropsy on all birds in your flock, it is possible that they are carriers. I would just treat the sick bird and go from there. Hopefully, no other get sick. MG can pass through hatching eggs. If you would like to get a sick bird tested, this company will test for all respiratory diseases with swabs sent to you for anout $90:
http://www.zoologix.com/avian/Datasheets/PoultryRespiratoryPanel.htm
Here is a good article about MG:
https://extension.umd.edu/sites/ext... Mycoplasma gallisepticum (MG) Infecti....pdf
http://www.zoologix.com/avian/Datasheets/PoultryRespiratoryPanel.htm
Here is a good article about MG:
https://extension.umd.edu/sites/ext... Mycoplasma gallisepticum (MG) Infecti....pdf