Any place where the verb טמא (unclean) comes as a future imperative verb (like יטמא - shall be unclean or וטמא עד הערב – and be unclean until the evening) – it makes it unclean to anyone else. But when it comes as a noun טמא הוא (it is unclean) – sometimes it means unclean to anyone else and sometimes it means that it is invalid but not unclean and that it is unclean in the sense that it is sick and disgusting or the opposite of holiness. And Rabbi Akiva introduces this rule.