If they are fixing to come into lay, stress can trigger latent disease. They can also catch it from anywhere like others stated. It may not necessarily be the feed store.
If you want to help them through it, you can try the VetRX which can be picked up at feed stores usually. Directions in the package. Organic Apple Cider Vinegar in their water could help cut through mucous in their throat. I think it is a tablespoon per gallon. If they aren't used to it, you could start with half that dose. I would add some vitamin supplement to their feed or water also. I would change the water daily. Or you could just pick up antibiotics for their water. Plenty of stuff on the market for MG if that's what you think it is.
