My infant daughter has a dry spot on the back of her head"fungal infection"?

My 4 month old daughter has had a dry spot on the back of her head it use to be red too,she lost some hair just on that spot.Her doctor gave me a prescribed cream cuz she thought it might be cradle cap.jus recently they tested it and it was a fungal infection.HOW DO YOU GET THAT?!i'm a very clean person,house included.now i got an over the counter cream doc told me to get and a prescribed shampoo.can you give me info bout it?how long b4 it goes away?it itches her