Wed Sotah 30

The Gemara says that someone who eats tamai food becomes Tamai.

Tosfos quotes the reason for the rabbis to enact that for perhaps he'll be eating Trumah and he'll drink something Tumah. This would apply to even a Beitza. However, the Gemara in Eiruvim says that it's only by two Beitzim.

Also, we see that the gemara had a thought that it may be a Torah Tumah. But how can they have though that. This was part of the famous 18 gezairos of Bais Shamai that became accepted, and all knew what they were. So how can they thought it was a Torah law?

Tosfos answers: there was two steps of the gezairah. Originally they only worried that people would eat them one after the other, people will say they're touching in the stomach and is making the Trumah he ate afterwords Tamai. However, since all the food doesn't make it in the stomach (some is left in teeth and some is already digested) so it wasn't applicable if he ate an exact Baitza. Therefore they upped the Shiur to the next exact amount, to two Beitzim.

Afterwords, Bais Sahmi was worried that they might end up in the mouth together. This is applicable to a whole Beitza. Therefore they made the enactment more stringent and made him Tamai by eating one Baitza.