All humans have a harmless amount of yeast in our bodies, it lives in our mouth, bowels, skin, and of course for women, the vagina. The problems occur when this small amount of yeast becomes unbalances and goes into overdrive.
What usually sets it off is antibiotics (either the baby taking them or the mother taking them, and they pass thru the breast milk). If the baby had thrush, a form of east infection, it could wind up as a yeast infection on the bum as well, when it travels thru the digestive system and come out in the feces.
The absolute best way for this to clear up quickly is to keep the area dry. Change diapers more often, apply the cream and add a huge dollop of vaseline on top of it, spread all over....this will help prevent the actual skin from getting wet. Do this a few times a day, and it should clear up in a week if you are diligent with the cream and the vaseline.
I hope this helps, if you have any further questions, feel free to ask, it costs nothing to click reply. If this has helped, please remember to click the green accept button.