Prophetic Tumah
There are two major halachic ramifications when a woman became a nidah. The one that is relevant today involves the relationship between a man and his wife.
Tumah and Taharah
Retroactive Tumah of a Niddah in the Gemara
The rabbinic rule that food a woman touched in the 24 hours before becoming a niddah is treated as tamei, and how it explains Avraham Avinu's rolls.