A bird with good resistance may not show symptoms or only mild . I have seen birds that had mycoplasma have a very light rattle when breathing . Others have runny eyes and swelling of the facial sinus cavities when first infected .
Think of carriers like chicken pox in people . When you get it as a child and recover your body has suppressed the virus . It hides in ganglion nerve centers and goes dormant . It is called shingles when it reappears later in life . Often in older people who have weakened immune systems . Stress can also weaken your immune system . So you are a carrier but the virus is dormant . You are infectious when it reappears .
I noticed years ago that birds that recovered from coryza would often get it again in the winter ( stressed by very cold temps ) . They would then infect other birds .
From what I understand, and I believe this depends on the disease, my birds that are healthy are carriers of MG. MG will die off on our property if we destroy all our birds, but some other diseases, like Mareks, won't - they live on in the soil. Yes, stress will bring it out. There are so many diseases that chickens can get so in the end, I believe it is best to breed for resistance -keep only the healthy ones who never showed signs of illness. Also, I believe in not vaccinating as it only weakens the overall immune system. There are many ways to build up their immune system - high quality food, fresh water, sunshine, fresh air, deep litter in coop and run (healthy non-poopy soil= healthy birds), fermented feed for probiotics (and many other benefits). So, there is a lot you can do, and fairly easily, to strengthen the health of the birds. The "old timers" and the natural chicken keeping thread are great resources.
I only bring it up because I have two birds in my garage that are sick, they were shipped to me in Jan. as 3 month olds and a couple days after they arrived the symptoms showed up, they had some clear nasal discharge and swelling in what looks like the tear duct (I know they don't have tear ducts) There's no weezing, sneezing, coughing, or other signs of sickness. They are growing well, eating and drinking well. The boy was on penicillin for a week and that turned his nasal discharge from yellowish to clear. I waited a few days giving them probiotics and then treated them with denagard for a week. The nasal discharge seems to have stopped but the "tear ducts" are still swollen. I'm kinda stuck on what to do since I don't want to infect the rest of my flock. I have 12 birds that I got as chicks in April and they've never been sick. The two garage birds aren't getting worse, so I haven't been able to consider culling them since other than the minor swellling they seem to be doing great. The rooster has even started crowing this week.....
What is the expected recovery time for chickens I guess. I think the problems started from the amount of DE that was put in their transport boxes and their run (our fault). We've since removed all the DE, could they still have particles irritating their sinuses after a month?
If the nasal discharge was tinted, and cleared when you administered penicillin, then they came with infection.
Personally I'd never integrate them into your flock. It's just not worth the risk.