Buttermilk is not one drink. There is traditional buttermilk which is what is left over after churning cream into butter. It is made by fermentation of the milk sugar by Streptococcus lactis (Milk was left to sit for the cream to separate, and allantoin from that same bacteria is part of what gives butter its flavor. but the reduced pH of the milk would also help the cream separate and give the butter better shelf life.). Cultured buttermilk is made by treating 1% or 2% milk with either Streptococcus lactis or Lactobaccillus Bulgaricus (the latter making the more tartly flavored drink).
Buttermilk is typically low in fat, high in acidity compared to normal milk and the casein, or milk protein has started to denature or coagulate, producing flecks of clabber. It is a common drink in India and Pakistan, and in some other locations, though Yogurt Drinks made using Lactobacillus acidophilus are also common in warmer countries.
Like most other milk drinks in the US, buttermilk is generally pasteurized, though there is a lesser need since the acid produced by the bacterial fermentation tends to inhibit bacterial growth.
Raw (unpasteurized) buttermilk cultured with Lactobacillus bulgaricus is used frequently, or in ombination with Yogurt, for stomach conditions. It is good for you. That made with Streptococcus lactis is also safe as the bacteria will not infect humans (The lactobacilli often live in symbiosis with us, where each benefits.)
EVEN if you are lactose intolerant (and about 2/3 of all adult humans are, especially those who trace their ancestry to areas where cow's milk is not used as food), buttermilk is OK. The bacteria have processed the milk sugar lactose into lactic acid, which does not require the digestive enzyme, lactase, to digest.