How does milk become curd?

The sour fermented milk product long enjoyed every where in the world. It probably didn't help that it was marketed under the slogan "Doctors recommend it". Today, it's a lunchbox staple, prized not only for its fresh, tangy flavor but also for the live bacteria most  yogurts contain. 

It's not that any bacteria can be used to make yogurt.  Initially Milk is pasteurized to kill any bad bugs and then cultured with two organisms approved by  Food and Drug Administration (FDA),The bacteria Lactobacillus bulgaris and and Streptococcus thermophiles, that convert milk sugars to lactic acid. The acid imparts flavor and curdles the milk, thickening it to a pudding-like consistency. Sometimes other bacteria, such as Lactobacillus acidophilus, are also added for their believed role in balancing bacterial populations in the human gut