Board Thread:Questions and Answers/@comment-26107879-20160111192627/@comment-24769684-20160111200655

Well the manga does answer these questions, but not for several hundred chapters. I recommend you read it to find out, but this is why if you want to know:

Fullbringers are people whose mothers were attacked by Hollows. Ichigo's mother was bitten by a Hollow creation in her youth

His Hollowfication is a bit more complex story. When Ichigo's mother was bitten by a Hollow, Isshin sacrificed something important to save her, allowing her to survive and Ichigo to be born. However, Ichigo broke the balance keeping his Hollow powers away during his Shinigami training.