Canadian pop star Justin Bieber has given up his meet-and-greet interaction with fans because doing so makes him feel "spiritually drained."

However, the Christian singer recently made an exception for one girl who is battling a life-threatening disease.

Sarina Costanzo,14, has been battling cystic fibrosis (CF), a genetic disease that causes persistent lung infections and progressively limits the person's ability to breathe.

Costanzo is having a difficult time taking pills and undergoing breathing treatments, and the only thing that motivates her to face each day with a brave face is listening to Bieber's songs.

"Having CF, it's hard to stay motivated to keep trying, and all the medicines that I have to take, it's really hard, and [Justin Bieber] just, his songs, just inspire me to just never give up on anything," the young girl told Fox News.

She requested to meet her favourite singer through the Make-A-Wish Foundation, and Bieber happily obliged. They met on April 6 when the 22-year-old pop star held his Purpose Tour stop in Pleasant Valley, Missouri.

Costanzo recalled that Bieber first introduced himself to her family and friends, and then proceeded to give her a huge hug. "I was just so nervous!" she said. "I was shaking so much. I just couldn't believe that my role model is right in front of me!"

Her emotions eventually got the better of her because she started crying. "I cried on his shoulder!" she said, "and he rocked me back and forth, and he was like, 'Aww, don't cry!' because I was, like, bawling, and it was really amazing."

When she finally regained her composure, she requested to have her picture taken with Bieber. The singer did not hesitate, took her phone, then took their selfie. Before leaving, Bieber even kissed Costanzo's head.

The girl's mother Loura Miller is very grateful to Bieber, saying she was touched by the way Bieber talked to her daughter. "She was hugging him so tightly, and he never pulled away. He just let her hug him until she was done, and it was a good five minutes," she said. "Even though he doesn't know her, he probably forgot those five minutes after we walked out of the room, but the impact it has on her will last forever."