Keep in mind folks that Speckledhen speaks for her own flock, right or wrong, I will not argue that as we have had our own conversation regarding these type issues.  
When these birds have Mycoplasia type infections, they do not get rid of it. Its like Herpes in humans- the cold sores on our lips. It is contagious forever, although our bodies are able to build up immunities, when you are contagious, you don't kiss someone.
At the same time, if you was to get a secondary infection, then you would treat it, like we do our birds (terramycin). If they have a cough, sneeze, and no real wheezing, keep a close eye, separate them feed them good food, spoil them a little and when they clear up, swollen eyes, and symptoms, then toss them back in. More then likely your other girls have already been exposed. Some birds never show signs, that is why I feel this is a silent killer of our poultry flocks. Even if we eradicate every one of the chickens in the country, our soil, wild birds are contagious and then we have it all over again from the "healthy" stock that comes from these hatchery eggs and also from a egg swap. Yes, we BYCers are spreading this to each other from non-tested birds. NPIP does not cover these other diseases, so a NPIP flock does not mean a whole lot at this point. NPIP is for Pullorium/typhoid, it is mostly non-existent! The job was done by the testing, yet now we have further evolved into the next disease's that need eradication. 
I feel like a broken record, but this is running like wild fire, vaccines may be the answer, but it will create a contagious nation.  
If you show or take birds to the fair, consider them infected with any or all of these conditions, this is where these virus' reared their ugly heads. 
Practice BioSecurity- that is not just for the new birds! This is where you place a boundry around your hen pen, if you mix a bird with anothers birds, it may not show signs of health concerns, but once you bring it in, boom, guess what...you are now ill, especially from commercial layers that have had all their shots- they are immune and spreading the things that they have been innoculized for. This is why the term All in All Out, so you do not cross contaminate.
I'm sorry, I am so amazed at how many folks are screaming sneeze, cough, chronic respritory disease, mycoplasia G. or S. 
Let us spread the word!