Why do you need to heat milk to make homemade yogurt?

Question

All the homemade yogurt recipes I have found online say one must heat milk up to 180 degrees, then cool it, before putting it into a yogurt machine.

Why? If I don't heat the milk, will I become ill if I eat the yogurt? Or is it more a matter of the yogurt not thickening or setting properly? In other words, is it more of a physical health issue to heat milk, or a recipe issue?

Answer 1

to kill the useless bacteria, so have to heat until 80 degress Celcius, then cool it down to 45 degrees celcius, because the yoghurt bacteria will live in that condition