Kirsty Carrington thought nothing of giving her newborn son a loving kiss.
However the doting mum had no idea the simple peck would leave eight-week-old Ole-John fighting for his life.
Kirsty was devastated when just 24 hours after kissing her baby last month the little boy fell ill with soaring temperature and breathing problems.
He was taken to hospital where doctors warned Kirsty they may not be able to save her baby.
Kirsty, 28, from South Yorkshire in England, was shocked when she discovered OIe-John was ill with a virus which had passed on from a cold sore on her lips.
"I'd never do anything to hurt my kids. I kissed my baby because I loved him and I'm devastated to think how close I came to killing him," Kirsty told UK's Sunday People magazine.
Kirsty has experienced cold sores since her teenage years and suffers outbreaks from time to time.
"I knew I wasn't suppose to have skin to skin contact during a flare up. But I made a mistake," she said of the day she kissed her baby son.
"I forgot the cold sore was even there as I kissed little Ole-John. And my mistake nearly cost my baby his life."
Cold sores caused by the herpes simplex virus can be dangerous, and even fatal, for babies. The virus can spread to the brain and other organs, potentially causing permanent damage.
Kirsty, who already has seven-year-old twin daughters Lilly and Kendra with partner Chris Nixon, said she felt helpless as her baby's temperature sky-rocketed about just one day after she kissed him.
When Ole-John's temperature reached 40.2C in the middle of the night, the frightened mum called emergency.
"They suggested I wait until the doctors' surgery opened in the morning but I insisted something wasn't right with him," she told Sunday People.
"His breathing was rapid, he was agitated and hot. So they called us an ambulance."
Doctors at the hospital initially believed the little boy might have meningitis due to his symptoms.
"Everything was so panicked and chaotic," Kirsty said of the first 24 hours in hospital.
"(Ole-John) was so agitated and I couldn't help him. He was struggling to breath and started jerking involuntarily. His eyes were firmly shut and he was making such a distressed sound."
Kirsty still did not link her cold sore with Ole-John's condition and doctors told her they would need to perform a lumbar puncture on the tiny boy to diagnose the baby's illness.
Two days after Ole-John was admitted to hospital the family was visited by a different doctor as the baby boy continued to fight for his life.
"He told us Ole-John wasn't responding to any treatment. He asked us if anyone in the family had suffered any serious infections or illnesses recently," Kirsty said.
"I suddenly jumped to my feet, shouting about my cold sore. I told him I'd gone to the chemist to have it checked.
"Panic rose in my voice as I admitted that I had. The doctor said he was in a critical condition but they could now try a different treatment plan."
Doctors discovered the herpes virus had travelled to the Ole-John's brain and he was diagnosed with with meningoencephalitis.
Meningoencephalitis involving the herpes simplex virus is fatal in one third of cases. Those who survive can face brain damage, sight and hearing problems.
The baby boy was given antiviral medication and within 24 hours his temperature came down and he opened his eyes.
"The first time he smiled at me, I burst into tears. I knew my boy was back," Kirsty remembered.
Ole-John was released from hospital six days after being admitted, however the long term implications of his illness are still not known.
Kirsty wants other to learn from her experience and is warning all parents to ensure nobody with a cold sore kisses their baby.
"I didn't realise how dangerous my kiss could be," she said.
"I kissed Ole-John because I love him, yet I nearly killed him. I hope my story serves as a warning to other cold sore sufferers – whatever you do, if you have a flare up, don't kiss your baby."