Why do beer steins have lids?

Aside from being the coolest guy at the bar, flicking open the lid of your ornate stein as you took a drink of your frothy brew, it is believed that the lid was implemented as a means of keeping diseased flies, possibly spreading the Black Plague, out of your beer. Apparently, if the flies got to the food you could simply not eat, but the beer could not be sacrificed. At least they had their priorities straight!