MB 20: Small amount of excrement - Even a tiny bit, because there is no minimum measurement. R' Akiva Eiger writes in the name of the Nezirus Shimshon that one does not have to keep away from human sick-up, even though it is repulsive.
MB 21: Saliva is thick - Then it is considered a covering, only as long as he can't smell anything. And if he puts water on it it does not help unless the water is dirty, such that you can't see the excrement through it. It is not comparable to excrement behind glass because glass is considered a proper cover [and water is not]. Therefore one should be careful not to say the Shema next to a bucket of water containing soiled childrens' clothes.