RIGHT now my kitchen is teeming with bacteria, and I’m doing everything I can to make them feel at home. They’re lactic acid bacteria, the ones that ferment milk into yogurt and buttermilk, cream into crème fraîche. I’ve been making all of these, as well as milk thickened with reputedly beneficial “probiotic” lactic acid bacteria. And getting to know viili, a Finnish fermented milk that reminds me of the Japanese soy product natto. It’s slithery.

I’ve made my own yogurt nearly every week for more than 10 years, beginning with a starter given to me by a friend from yogurt-loving India, and using the last spoonfuls of one batch to make the next. It’s a satisfying ritual of continuity and caretaking. And the yogurt is less expensive and better than anything I can buy. It’s free of stabilizers, sweeteners and waterlogged fruit, and it’s fresh tasting and tart, not sour. I start every day with a bowl of it.

Even if cultured dairy products aren’t part of your daily regimen, they’re worth making once in a while just to know how good they can be, and to experience the everyday miracle of fermentation. You stir a little starter into warm milk or cream, let it sit, and in a few hours the bacteria have multiplied a hundredfold and created a tart, aromatic, thickened mass, with many billions of bacteria in every spoonful.

A growing number of studies have found that some lactic acid bacteria do seem to offer health benefits, supporting the lore of traditional dairying cultures. The lactic acid bacteria are a group of microbes that share the ability to convert sugars into lactic acid, which suppresses the growth of their competitors. The lactic acid also causes the proteins and fat globules in milk to cluster into a continuous solid network, with the milk’s water trapped in its pores.