The problem is, penicillin kills not only the germs you have that you don't want, it also kills a lot of the germs you do want. Our body is filled with ga-zillions of little critters, ie. germs. Some are beneficial and some are harmful. Normally they live in homeostasis with a constant battle going on. Antibiotics kill lots of this flora and fauna, both the good and the bad. Now there is nothing to fight the yeast and keep it in check so it overpopulates. Voila! Yeast infection. Eating yogurt is a way of reintroducing some beneficial critters into our bodies that will fight off the yeast to a tolerable level.
Doctors say that penicillin may cause a yeast infection and many doctors prescribe diflucan along with it to prevent one. Although there may be several reasons of yeast infections, it is a well known fact that some antibiotics like penicillin may contribute to them developing. Read more at http://www.ehow.com/about_5070501_penicillin-cause-yeast-infections.html.