A mum was slammed for 'hitting' her baby. But she's actually spreading a powerful message.

A mum has shared a video that shows her “hitting” her infant daughter to bring attention to the harsh realities of caring for a child with cystic fibrosis.

UK woman Samantha Carrier posted a video of her daughter’s morning physiotherapy routine, which involves the parent repeatedly slapping baby Daisy Evan’s chest, on Facebook.

The Birmingham mother explained that the treatment was performed for 25 minutes, three times a day from when Daisy was just over two weeks old to try to shift mucus and prevent lung damage.

“I realise this is shocking if you don’t realise why,”  she wrote.

“It doesn’t hurt her. We are taught how to do it properly so it doesn’t.

“Generally she just falls asleep. Or she lies there smiling at me. It makes them feel better.

“It’s like when we have an irritating cough and we finally get it off our chest, how much better we feel. This is what it does for them.”

However, not all viewers got the message, with several posting critical comments questioning if the treatment, known as chest percussion, was necessary.

“You’ll break her ribs,” one commented.

“You’re doing it too hard,” said another.

Ms Carrier even received a death threat, according to BBC Midlands.

Thankfully, this was overwhelmed by the majority of messages of sympathy and encouragement from supportive Facebook users.

"She looks so content, gorgeous little doll. You're doing amazing. Don't listen to the silly people who having pointless opinions," a supportive commenter wrote.

"I take my hat off to you I think you are an amazing mum that is helping her beautiful baby I hope your little girl has a long and happy life," commented another.

Ms Carrier thanked commenters for their support.

"Unfortunately you can't educate everyone and although I have tried some people just don't get it," she replied.

"But we don't let it bring us down. We are a strong family with a lot of love and support."

The video, which has now had more than 149,000 views since it was posted in December, was uploaded to a Facebook page Ms Carrier set up to share her daughter's journey.

Ms Carrier said the page was also created to promote and raise funds for charity Cystic Fibrosis Trust and spread awareness of a "life changing drug being blocked".

On an accompanying crowd-funding page, Ms Carrier writes that her "heart sank" when Daisy was diagnosed with the condition that currently has no cure.

"I am not ashamed to say I didn't know how to cope with it all. I have had some very dark days. But one day you wake up and you realise this is it now."

"All we can do as a family is try to do our best by her and give her the best life we can.

"Myself and Rob have decided to try turn a postive. So we want to raise as much money as we can through various events for the Cystic Fibrosis Trust who research into new medicines and one day possibly a cure."