A staph infection can come and go-mine started with 2 small sores, cleared up for a bit, then I got a big one, cleared up again with antibiotics, then came back with an enormous life-draining horrendouly painful sore that I am currently fighting with the help of even stronger antiibiotics. It could be it was impetigo, but I doubt it. As I am reading up on this, it seems there is an antibiotic-resistant strain of staph that is running around the country as a very quiet but increasingly common epidemic. She could have picked it up any number of different ways-it's no longer an infection that aflicts only those with a weakened immune system or those already in the hospital. Whatever you do, DO NOT slack off on the antibiotics the doctor prescribed, keep the wound covered AT ALL TIMES unless you are draining it, and WEAR GLOVES anytime you are changing her diaper or cleaning or dressing the wound. This is an unfreakinglybelievably, incredibly painful infection I would wish on no one in this world, and it is unbelieveably contagious-we think I picked it up from my mother, and we think she picked it up from cleaning houses.
It is very, very common to have clusters of it in families. It often starts out with sores inside your nose (painful all by themselves)-if you break out with those, RUN, DO NOT WALK, to yuour doctor and beg him for antibiotics BEFORE you get a boil on your skin.