Without yeast, there is no warm bread, cold beer or creamy cheese, no wine or whiskey. A world without yeast is hardly a world worth living in, culinarily speaking.
Yeast is a fungus, and as strange as it may seem, that beautiful glass of Malbec you're drinking is a fungal concoction. There are many different kinds of yeasts, and while some create gustatory pleasure, others work toward a goal like diaper rash. (These yeasts are, perhaps, the evil stepsiblings of the cheese kind.)
Yeasts are everywhere. You can find them in soil and water, and on plants and people, too. And while most of the time these organisms contribute happily to the little colonies of beings that live in harmony on your skin, in your mouth and in your innards, disruptions in their ecosystems do not go unnoticed. That's when people get conditions like oral thrush.
The yeast behind oral thrush is Candida albicans (although sometimes Candida glabrata or Candida tropicalis make a guest appearance). It's the same great yeast that brings us vaginal yeast infections and the skin infection candidiasis. (Yes, oral thrush is a nicer way of saying "yeast infection of the mouth.")
But for most people, it's not so bad. Find out what oral thrush looks like on the next page.