Basically I guess I’m asking if this is a hopeless situation and all of my chickens are going to get sick and die spontaneously down the road, or if there’s a chance that we could get lucky and they all end up doing okay?
Hi, welcome to the forum, glad you joined.
Is it possible you have a problem? Yes, anything is possible. Is it likely you have a problem, not really. People do a whole lot worse than you have and usually are OK. Worry is interest paid before it is due so quit worrying.
Quarantine is a powerful tool when done right for the right circumstances. Chickens can catch diseases form other chickens. But these diseases take time to show themselves so you quarantine them for a while after they have been exposed to other chickens. If that breeder has not been moving new chickens into his flock or taking his chickens somewhere like a show where they have been exposed to new chickens they have been in quarantine at their place, just not yours.
The big danger comes in when you get your chickens at a swap or auction. To me that's when you really need a quarantine. Even then a lot of people take their chickens to these events and bring chickens home regularly or swaps and auctions would not happen. Many of those people do not quarantine. Sometimes they do get serious problems but most of the time they don't. If you were guaranteed serious problems those events would not exist. The most common problems from swaps and auctions are probably mites, lice, or worms, maybe Coccidiosis, and you can treat for those.
Some flocks develop flock immunity to something. That could be your flock, it could be the breeder's flock. Coccidiosis is a good example but there are others. Since they have developed immunity to it no matter how long you quarantine them they are not likely to show symptoms. Quarantine doesn't help a lot with this. If the new chickens get sick and your old ones don't your existing flock probably infected them. If your flock gets sick and the new ones don't they infected yours. So you do need to pay attention to all of them and treat if necessary. But think about this. If either your flock or the new flock are sick, they are not dead. There is a really good chance the ones that get sick won't die either, especially if you treat them.
You do not get guarantees with this. Anytime you bring in new chickens you have a chance of bringing in something, whether you quarantine or not. Sometimes that can be serious. But your question is whether you are in a hopeless situation. Not even close. The odds are really good absolutely nothing bad will happen.