Seems to me that most flocks have it. Like you say, you can cull them all, fumigate everything and get new birds only for a crow to come in and give it to them again. It would be better in my view if anti-biotics were avoided where possible to allow natural resistance to build up in birds. It appears that once they've had a bad episode and got over it, they are much less likely to suffer again in the future. As Chris says, good care and conditions is probably the best solution.