You are right, and most likey, they will carry an overload, just not showing signs yet. I had mine start with a few and then all...so best to treat all, as it is very contagious! This parasite, in young birds is a bugger of a thing. Took my birds hard and fast, after being stressed. I was away for a bit one day, they were fine in the morning, got back, and noticed a few kinda runny poo's, by that night, all had it! Once you get them back on their feet, so to speak, clean out their bedding and do a good scrub down too. I am doing mine on Thursday, since that is when our run will finally be done and I can let them out. Plus keep their litter topped off. I cleaned out their bedding that first night and added new, then topped with straw, although I don't like the straw as much as they do. I can't see their poo as good and have to go fishing for it...they seem to be hiding it...not really..just being chickens and scratching....I am topping their bedding up, as needed, to keep them off the contaminated poo's, until the thorough cleaning is done!I have been doing the "Save a Chick" Vitamins and Probiotics as recommended on the package.
I'll save what I have now for later on down the road.
It's just the one chick that started looking in bad shape... the rest of them (as of this post) are still flying around and pecking at anything that they think might be new or something they haven't tried yet....
But from what I've read, it doesn't hurt to give them the medicated water for prevention.