1. The Gemara explains why the Kohen Gadol needed to be separated from his wife for seven days before Yom Kippur.
2. Most people who require immersion must immerse during the day.
3. The Tum'ah of a man who has an emission is like a person who touches a dead Sheretz.
4. The Tum'ah of a man who has relations with a Nidah is like the Tum'ah of one who comes in contact with the dead.
5. There is a dispute about whether one may serve in the Beis ha'Mikdash while he is Tamei from contact with the dead.
A BIT MORE
1. This is in order to prevent him from accidentally having relations with his wife when she is unsure about whether she is a Nidah, which could cause him to be Tamei for seven days and, therefore, unable to serve in the Beis ha'Mikdash.
2. A Nidah and a woman who gives birth immerse at night.
3. They are Tamei until nightfall and are considered a Rishon l'Tum'ah.
4. Not only is he Tamei for seven days, but he is considered an Av ha'Tum'ah.
5. Rav Nachman: One is allowed to offer a public Korban even if he is Tamei (his Tum'ah is "Hutrah b'Tzibur"). Rav Sheshes: One is allowed to do so only because the necessity of bringing the public Korban overrides the prohibition of serving while in a state of Tum'ah (his Tum'ah is "Dechuyah b'Tzibur").