Kelim, Chapter Eight, Mishnah Five
1) If a rooster that swallowed a sheretz fell within the air-space of an oven, the oven remains clean;
2) If the rooster died, the oven becomes unclean.
3) If a sheretz was found in an oven, any bread in it contracts second degree impurity since the oven is of the first degree.
Section one: A source of impurity that is contained within a living being, such as a rooster, does not spread out and cause impurity to the vessel in which the rooster is found.
Section two: However, if the rooster dies, it can no longer contain the impurity of the sheretz and the impurity of the sheretz will escape and defile the oven.
Section three: When a sheretz is found in an oven, contracts first degree impurity. The oven then imparts its impurity to the bread, which now has second degree impurity. We do not consider the air of the oven to be impure such that it directly defiles the bread. This would cause the bread to have first degree impurity. As we proceed we will learn about the practical differences between first and second degree impurity.