Starting sourdough is way easy... if you start with fresh-ground wheat, the wheat berries already have all the yeast they need, rather like grapes for wine. Even if you don't have fresh-ground wheat, the yeastie-beasties are all around in the air as well. Usually molds don't get a hold on the starter unless it's very humid, or you've been neglecting the starter and the yeasts are not as vigorous. We always found that if we baked something at least once a week and fed the starter every other day on the non-baking days we always had a vigorous, bubbly starter. And if you decide you want to start fermenting your animal feeds- a glop of the starter is a wonderful way to get it going.