MG, or Myco as it's commonly referred as, is the last thing you want to get! It's a carrier for life as other members have said...we brought in 5 new birds in the Spring and we noticed one looked a bit snuffly but didn't take much notice of it. After they'd been in their new home for a few days (separate from our main flock) they all started to look snuffly and had swollen eyes etc, all the symptoms that Womble has mentioned but we didn't know it was MG at that point.
Then our main laying flock started to go down with it! Devastating, cos they don't lay while they're suffering with the worst of it!! One by one they all caught it - fortunately we researched on the net and decided that it was definitely MG then managed to get the Tylan powder into their drink (from the vet).
The best bit is that none of ours died!! We did isolate some of the worst birds to reduce stress for a few days.
The worst bit is that we've had to get rid of our entire flock, including the new ones we'd brought in (which we'd planned to breed from)

They'll be carriers for life even if they appear healthy afterwards, and apparently the symptoms can come back, brought on by stress from moving/changes in environment/weather etc. Also, you can't hatch from their eggs as it will even pass thru the shell and infect the chicks!!
The really annoying thing is that it's v difficult to tell if you're buying birds that are infected (although it was quite obvious with ours, just that we didn't know what to look for!!).
As Marches Farmer says, MG can pass through dust, poo, bottom of wellies etc! So if you're passing between an infected flock and a clear flock you need to leave at least 2 hours between visits so that the infection doesn't spread.
When you get rid of all infected birds, leave at least 2 weeks before introducing new ones to the house/run and thoroughly disinfect everything.
As you can see I did quite a bit of research on this!!
The long and short of it is - don't get MG if you can possibly help it!!!! It's a nightmare!