A Youtube video featuring a 4-year-old girl in a hospital room singing her favourite Mandisa song, "Overcomer", is winning hearts on the social media.

The little California girl is fighting a rare blood disease, severe congenital neutropenia, diagnosed at birth. She has reportedly spent more than 400 days in the hospital.

Leah Carroll received her second bone marrow transplant at UCSF Benioff Children's Hospital in Oakland last year and she is home now from the hospital.

"She does encapsulate that song, she is an overcomer," Leah's mother Lindsay Carroll said. "It's amazing to see what a little 30 second video can do. I mean it just blew up."

"I like to say that all the pain we went through... when we see the inspiration that's been taken for our story," Lindsay said.

"She's a farm girl and she's a teacher by day, and she's in the Navy Reserves–it's cool for me to think that Leah has a little bit of that in her," she added.

Leah has even met singer Mandisa, and she is soon set to meet the woman who has donated her bone marrow, a teacher named Holly Robinson.

Leah and her family are squarely focusing on helping other kids and adults who need bone marrow transplants especially in the mixed race or minority communities.

Dr Mark Walters, M.D of Benioff Children's Hospital, said, "If you're white and from Northern European background, the likelihood of finding a match is 70 percent. If you're African American, it drops to 19/18 percent," reported by ABC7 news website.

Leah has become a poster child for BeTheMatch.org which is trying to increase the number of prospective donors.

"She has no idea. I told her one time, honey you're changing the world and she goes 'I know,'" her mother said.