When we had trouble the Defra and our local vet both agreed that they were given Baytril as a broad band antibiotic until they post mortem a live bird and grew cultures in the lab to find out what exactly what we had, it was salmonella and as in any creature as well as humans you get them that clear of the disease and them that are carriers for life.
They were given regular droppings tests to see if it was in the other loft and hen coops, because we had hens this was stipulated rather than precautionary. Although I chose to cull all the birds in the infected loft, all because we never at that time isolated new stock, the bird that brought it in was from a well known fancier who turns out to less than honest in his practices. :-/