We have learned the hard way when we first started that Salmonella can be treated but there are two possibilities as there are in humans as well, it can be cured or it can stay as a carrier for life. Only tests can determine this, almost every creature has salmonella at some level, if you do not cook meats in particular poultry you can be become ill with salmonella so it has to present in the first place
We lost a whole loft of birds because we brought one in without quarantining it, it was from a well known fancier and looked really healthy and we found out too late that this was our problem, we had to blow torch the loft and we burned all the nest boxes and perches that ###### had made and made totally new ones
We have learned a lot about it through DEFRA and vets as we keep chickens as well, sadly as I say the hard way, we will never eradicate it, if your birds land anywhere outside your loft and a seagull, crow, sparrow, squirrel or countless other creatures has fouled there then you are open to contracting it