From what I understand, MG is a life-long carrier thing. IB sticks around in the flock for about a year. 
MrsB

MrsB
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My birds got Infections bronchitis last year.
Some were really bad and could hardly breath.. just standing with their head in the air and gasping and struggling to breath with loud raspy breathing and coughing.. Others were not so bad.
Weird thing was the worst looking ones got better the fastest.. and the ones that showed only mild symptoms stayed ill for a long time.
I had about 20 birds.. they all recovered fine with no treatment.. only clean, stress free environment and proper food and clean water 24 / 7.
There was a sharp reduction in egg production.. but after they recovered they went back to laying eggs the same amount as before.
I introduced new birds several weeks after mine got better... and the new ones never came down with it.
People often say to cull birds with any respiratory diseases as they stay carriers and sick even through they show no symptoms... but for people keeping a few birds in their garden I don't think you need to cull.. only if you are large scale commercial business.
The birds may stay carriers.. but they will have no symptoms and they are not 'sick'.. they don't feel unwell or behave any differently to a chicken that never got the disease.. they don't feel sick, so I don't see the need to cull them.
My birds had several disease over the years and I never culled any. Once they recover they are stronger and not likely to get that disease again. I never noticed any permanent reduction in egg production or condition of the birds themselves.