U.K. mom Claire Henderson took to Facebook to warn other parents about an unlikely danger facing newborns -- a simple kiss from a well-meaning visitor.

Henderson posted pictures of her month-old daughter Brooke with visible cold sores on her face and around her lips. She says Brooke contracted herpes simplex virus 1 (HSV-1), a version of the herpes virus transmitted orally, after getting a kiss from a hospital visitor who did not know she was infected. And while a cold sore might not seem like a serious health problem in an adult, to a vulnerable newborn the infection can be serious.

"Please share this with every new mum and pregnant woman you know," she wrote on Facebook. "Before 3 months old, a baby cannot fight the herpes virus. If a baby contracts this it can cause liver and brain damage and lead to death."

Please share this with every new mum and pregnant woman you know... COLD SORES CAN BE FATAL FOR A BABY. Before 3 months... Posted by Claire Henderson on Tuesday, September 15, 2015

It is very uncommon for a baby to contract HSV-1 from this kind of contact, Dr. Harvey M. Friedman, professor of medicine at the Perelman School of Medicine at the University of Pennsylvania, wrote in an email to CBS News.

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Most individuals contract HSV-1 as a child or an infant when the virus is being spread through skin-to-skin contact from an infected adult. Sores might not be visible on the skin of the infected person, according to the American Academy of Dermatology.

The other version of the virus, herpes simplex type 2 (HSV-2) is contracted mainly through through skin-to-skin sexual contact, and about 20 percent of all sexually active adults in the United States carry that virus.

Like Brooke's case, HSV-1 is generally passed on through kissing, sharing an object that comes into contact with a person's skin, like a razor, or simply touching someone else.

When a young child gets the infection, they generally experience fever, swollen and tender lymph glands, and mouth sores, according to nonprofit healthychildren.org. The sores will typically heal gradually over the course of a week or two.

What should a parent do to safeguard against their child contracting this virus?

"Avoid direct contact with baby if the visiting person or parent has a fever blister (cold sore = herpes) outbreak," Friedman wrote. "In the case with Brooke, the contact person did not know they were having an outbreak of herpes. In that case, there is virtually no way to prevent this rare event from happening."

As of Monday afternoon, Henderson's post has been shared more than 46,500 times on Facebook, and has made the rounds across other social media sites.

"I know this sounds like I am scaremongering," she wrote, "but if my friend had not told me about this my baby girl could have been seriously ill."

Henderson went on to write that baby Brooke is now fine and that all of her tests "came back clear."