Because of a kiss from her mother, baby girl Bonny almost lost her life.

A Perth mum has shared the terrifying story of how kissing her daughter almost caused the little baby’s death. It was all because she was getting a cold sore, not that she even realised it at the time.

Helen Farrell has told Channel 7‘s Today Tonight that when she and husband Russell Green took six-month-old Bonny to the UK, she got a cold sore the following week. She believes it was due to the air conditioning on the plane.

Like most mums, Farrell has always given her baby plenty of kisses.

“Kiss her face, kiss her head, give her raspberries on her belly, kiss her feet… I’m her mum, I want to kiss every inch of her,” she says.

But once she knew she had a cold sore, she stopped kissing Bonny.

"I wouldn't kiss her if I had an open cold sore, which I don't think anybody would do."

When Bonny developed a sore on her ear a few days later, her worried parents took her to the doctor, twice. But they were sent home with antibiotics.

It was only when Bonny suffered a seizure, lasting 20 minutes, and was rushed to hospital that Farrell and Green found out how serious the situation really was.

The sore on Bonny's ear. Image via Today Tonight. 

"The doctor came in and said, 'Is there anyone who's been in contact with her that had a cold sore?'" Green remembers.

It turned out that Bonny had contracted the herpes simplex virus from her mum's kiss, which got into her brain, causing viral encephalitis, or swelling of the brain.

She survived but was left with right-sided hemiplegic cerebral palsy, meaning she had lost movement on the right side of her body, as well as part of her left side.

“You just don’t expect that a kiss from a mum to her daughter can almost take her life away,” she said.

Cold sores can be contagious before they appear or start to tingle. The herpes simplex virus can be spread through saliva, and it can pass right through babies' fragile skin.

Farrell says she felt devastated. "I'm her mum, I'm meant to protect her. Mum’s kiss is meant to heal things, not break things."

"I feel very guilty but I've had to forgive myself. I can't live for the rest of my life blaming myself for it. It's one of those things. I've been reassured and reassured that it's just pure bad luck that it happened to us and to Bonny."

Bonny is now undergoing rehabilitation. Green says they're hopeful she will make a "pretty good recovery, if not a full recovery".

"You've just got to be a believer," he adds.