Liz Buchanan BVSc MRCVS
Hello! It's boring, but I have to write it: we always advise asking for first hand veterinary information before buying a pup with a known illness. In this case, if this was indeed a bite wound, then bacteria from the (biter) pup's mouth wound have been injected deep into the tissues by the action of the bite. They could sit there for a day or two but of course the fascial tissues are warm, with a good supply of protein, so any bacteria would multiply fast causing an infection, access or, more superficially, cellulitis. Bathing abscesses almost never sorts them out because the infection is well below the surface. Whatever this is, it is a great idea to show it to a vet asap. Mention how young the pup is and that it's a possible bite wound when you call. You may want to consider the emergency vets for triage.
Best of luck to all of you and please do let us know how you get on.