User blog comment:TheOriginalVampireKlaus/Lydia may be the reptilian shifter/@comment-49.248.7.26-20120619085942

Well no, it is explained that in the rare case, an infected person's body may entirely reject an infection and they neither die nor turn.

Also, its not Lydia because the silhoutte they sometimes show just before the lizard shifts (when it killed Isaac's dad, when in the house with Allison) is definitely not Lydia, it is a sleek figured male. They just trying to throw red herrings at the viewers by suggesting Lydia or Jackson.