זיהום הנחש

Another term that is often used beginning with בראשית is the זיהום הנחש, or the pollution of the serpent. The Kabbalists believe that the evil serpent actually had relations with Eve, and this polluted her seed.

This pollution carried on for many generations. Every family was affected by this pollution. When the Torah gives the chronology of each generation, it only lists the one child that was not affected by that pollution.

Therefore, we see why Cain, Yishmael, Eisav, and others were problematic individuals. The Kuzari mentions that it was not until Yakov Avinu that all twelve of his sons were not affected by this זיהום. Because of this, Hashem decided to turn these twelve holy sons of Yakov into the nation of Israel. This is what is known as the זיהום הנחש.