Impurity Communicated in Swallowing an Item - Tumat Giveeyah לפסול את הגוייה
Whether this form of impurity is rabbinic or biblical, as far as deciding leniently/stringently in terms of consumption of the shiur [minimum significant amount]: Yoma 80b
How much one must eat in order to contract impurity: Eruvin 82b; Ketuvot 64b