Most likely the rabbits had upper respitory infections. This can be treated with antibiotics but is sometimes difficult to treat and could requires long, high doses of antibiotics.
I would make sure I have washed my hands and changed my clothes before interacting with my rabbits after visiting. You have no way of telling which bacteria is causing the infection. One very common one is pasturella which is carried in the rabbit's system naturally and comes out when the rabbit is stressed or the immune system compromised. But, it can be caused by quite a few other bacterias as well.
If your rabbits are healthy and not stressed, chances are they won't pick anything up, but better to stay on the safe side and make sure you wash up after the visit to the breeders.
-Dawn