Redmolly, if my vet advised that i would be changing my vet!! This is a classic example of vets not knowing enough/cant be bothered with treating out girls. We pay for the treatment, we need to demand better services from the vets.
Here here Lizzie! What crass advice must have been the trainee on call on a Sunday.
MG has many forms from mild to severe and stress can enduce an outbreak in a dormant condition which none the less can be serious and not to be taken lightly. Ongoing management is needed even when the visible symptoms disappear and a half responsible vet should have advised yoiu. MG is vertically transmitted from parent to chick via the egg and carriers should not be breed from.
MG itself is not particularly serious and rarely causes death however it is usually complicated with several other infections most frequently Escherichia coli to provoke chronic respiratory disease (CRD). In this case it looks like this was a mild infection which wasn't complicated.
The following is a quote from the Poultry Diseases Network,
"MG may remain dormant and cause no disease until the chicken undergoes some stress, so the MG itself is not a killer, in fact, even morbidity is not great. However, an outbreak may be followed quickly by many secondary infections, and it is these that do the damage (5)."
If you are of a medical bent the complete article is worth a read.
http://www.poultrydiseases.net/online/index.php?option=com_content&task=view&id=14&Itemid=26HF