Yeasts are a unicellular fungi, and grow in lots of different places. Yeasts are found on wide variety of plant leaves and flowers, in soil, in water and in your gut. They are partly responsible for your body odour, and yes, thrush was most certainly period (Candida albicans causes that). They're used as a raising agent in cooking and it makes your brew ferment to produce ethanol. Yeasts are also very good for you, and are rich in folic acid (good for pregnant women), vitamin B and niacin.
Saccharomyces cerevisiae is the species of yeast most commonly used in your brewing and bread baking. They ferment rice, wheat, corn and barley sugars. The yeasts that ferment your wine are commonly found on the plants in the vineyard anyway. It's believed that no one used to intentionally used to add yeast in the goode olde days, but because grows everywhere it would breed in the cracks of the brewing barrels and other brewing equipment. Yeasts don't particularly like being heated above about 40C and will tend to start to die at about this temperature.