Beer that is exposed to the surrounding open air to allow natural/wild yeast and bacteria to literally infect the beer, are spontaneous fermented beers. One of the typical yeasts is the Brettanomyces Lambicus strain. Beers produced in this fashion are sour, non-filtered and inspired by the traditional lambics of the Zenne-region. This brewing method has been practised for decades in the West Flanders region of Belgium.
It's the same process of making sour dough breads...
Yeast floating in the air innoculate the moist dough.
Viola! Music hath charms to tame the wild yeast?
Wild yeast, wild grain, wild hops, water, and wild honey = wild nights, for those who on the honeyed dew hath fed.
Like sourdough, this gives sour beer?
Next thing you know, you have Wally Cox singing "There's A Tavern In The Town"...soon followed by Slim Dusty complaining about a "Tavern With No Beer".
Spontaneous Fermentation? Is that like The Immaculate Transubstantiation?.............