Reoccuring yeast infections

I've been having white and clear discharge and fishy smell down there for 2 months. I wash everyday with water. Finally went to the doctor to get it checked and they told me I have a yeast infection and it's most likely caused by urine. I don't understand how can you get an infection from urine. Everyone would have infections because everyone urinates lol... Last year I've been told the same thing, that I have a yeast infection when I didn't even have symptoms. What causes yeast infections and how can I prevent them? How dangerous are they?