give everyone who has signs a dose, one dose should do it for them
but for the one who is worse off give a second shot 24 hours later at half the dose of the first shot. Unless you see improvement. Doing it this way will keep the antibiotic in his system longer and help him fight it.
yogurt or probios would be good as already suggested.
I had the same thing run thru my herd awhile back when I got new goats and everyone got a shot and some got a shot the second day. Everyone got better thankfully. Anybody weezing I would think about doing a second shot tomorrow. That is what I did.