<PERSON> didn't know that he'd created a horcrux out of <PERSON>--after making so many before <PERSON>, he'd rendered his soul very fragile, by repeatedly ripping it into pieces to hide it. When <PERSON>'s protection over <PERSON> prevented <PERSON> from killing him, <PERSON>'s badly-damaged soul ruptured again, and the fragment that broke off hid inside <PERSON>'s own soul, as he was the only living thing in the house, and <PERSON> hadn't done the specific ritual to make an object into a suitable home for a horcrux. <PERSON> wasn't aware of the damage, because so much else was going on--he'd lost his body and was weak ("weaker than the meanest ghost").
Chamber of Secrets pg 245: "'You can speak Parseltongue, <PERSON>,' said <PERSON> calmly, 'because Lord <PERSON>--who is the last remaining ancestor of <PERSON>--can speak Parseltongue. Unless I'm much mistaken, he transferred some of his own powers to you the night he gave you that scar. Not something he intended to do, I'm sure...'"
Deathly Hallows pg 551: "'... Tell him that on the night Lord <PERSON> tried to kill him, when <PERSON> cast her own life between them as a shield, the Killing Curse rebounded upon Lord <PERSON>, and a fragment of <PERSON>'s soul was blasted apart from the whole, and latched itself on to the only living soul left in that collapsing building. 773
Part of Lord <PERSON> lives inside <PERSON>, and it is that which gives him the power of speech with snakes, and a connection with Lord <PERSON>'s mind that he has never understood. And while that fragment of soul, unmissed by <PERSON>, remains attached to, and protected by <PERSON>, Lord <PERSON> cannot die.'"
Deathly Hallows page 568: "'You were the seventh <PERSON>, <PERSON>, the Horcrux he never meant to make. He had rendered his soul so unstable that it broke apart when he committed those acts of unspeakable evil, the murder of your parents, the attempted killing of a child. But what escaped from that room was even less than he knew. he left more than his body behind. He left part of himself latched to you, the would-be victim who had survived."
Note: these are the page numbers for the 2007 Harry Potter hardcover boxed set, they may vary slightly from edition to edition. 773