A newborn girl is believed to have died after being kissed on the head by someone with a cold sore.
The virus is thought to have entered Mariana Reese Sifrit's body and passed to her brain causing deadly swelling. At barely a week old, she was taken to hospital by her parents when she stopped feeding. But she suffered organ failure and died earlier this week.
Her case has prompted warnings to parents about the dangers of letting friends and relatives kiss their babies in case they pass on a deadly infection.
Mariana is believed to have contracted encephalitis, a life-threatening condition in which the brain becomes inflamed, after coming into contact with someone suffering from a cold sore.
Most cases of encephalitis are caused by the herpes virus which is also responsible for cold sores. Both of Mariana's parents tested negative for the virus, they said.
However herpes can spread through a baby's eyes or mouth putting them at risk if they are kissed on the face by someone who is infected.
British experts say that although such cases are very rare,' parents should avoid allowing babies less than a month old to be kissed on the head.
Mariana died on Tuesday at the University of Iowa Children's Hospital, in Iowa City in the US.
She became ill just two hours after her parents' wedding which was six days after her birth on July 1. According to her mother Nicole Sifrit, 33, she stopped feeding and wouldn't wake up.
Initially she was taken to Blank Children's Hospital in Des Moines, close to the family home, then later airlifted to the larger University of Iowa Children's Hospital.
Her organs ultimately failed, and she died in the arms of her father, Shane, at just 18 days old. After Mariana's death, Mrs Sifrit warned other parents: Don't let people kiss your baby and make sure they ask before they pick up your baby.'
Professor Will Irving, a virologist at the University of Nottingham, said parents should avoid letting newborns of less than four weeks being kissed on the face or head.
At this stage of life it's potentially catastrophic,' he said. It's such a tragedy and it is very difficult to diagnose.'
The condition causes flu-like symptoms including a headache or fever, but these can progress to confusion, loss of movement, fits or loss of consciousness.
Patients require urgent treatment and often have to be placed in intensive care and given anti-viral medications or steroids.
About 4,000 Britons contract encephalitis each year. Irving said the odds of a baby catching the condition from someone with a cold sore are approximately 1 in 200,000.
Most cases are caused by the mother having genital herpes and passing the virus on to the baby during the birth. But a handful are contracted from someone with a cold sore kissing the baby including their own parents.
He said there had been at least one British case a baby who died from the illness several years ago in Nottingham. Doctors in the US had reported that Mariana had contracted meningitis from the cold sore.
But Irving said that it was almost certainly encephalitis. The condition is similar but affects the brain itself, rather than the surrounding membranes.
Babies less than a month old are most susceptible as their immune systems are not fully developed.
On Tuesday, Mariana's mother posted a message on Facebook which read: She is now no longer suffering and is with the Lord. Thank you to everyone who has followed her journey and supported us through this.
In her 18 days of life she made a huge impact on the world and we hope that with Mariana's story we save numerous newborns' lives. RIP sweet angel.'
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