Do a search on brucelosis, trichomoniasis, vibriosis, genital warts, BVD, Johnes, just to get you started and rethink carefully before making the final decision.
Other than the obvious health risk there is also another thing to consider. Bulls that are moved from herd to herd will sooner or later encounter less than perfect fences, this might just give them the idea that they can get through a fence if they want to. Face it if a mature bulls wants to be on the other side of the fence the fence itself is hardly going to slow him down. So part of the young bull's education is to respect fences since he learns from calfhood that he can't get through a fence and tends to remember that if he only encounters good fences, it will take only one cow that don't respect fences and a few poor fences for encouragement to teach him that he can get through and create a whole new problem for you.