Claire Henderson’s daughter, Brooke, was born about a month ago, and many visitors stopped by to meet her.

Henderson told BuzzFeed News that one of the visitors carried the herpes simplex virus 1 (HSV-1), which causes cold sores and can be transmitted via oral-to-oral contact.

She said the visitor didn’t appear to have a cold sore at the time she held Brooke. However, HSV-1 can be transmitted through saliva and surfaces in or around the mouth as well, according to the World Health Organization.

While feeding her daughter one night, Henderson said she noticed Brooke had a cold sore and swollen lips.

Claire Henderson

She said she knew she had to take her baby to the hospital immediately.

“My friend had shared an article with me on Facebook when I was four months pregnant on the dangers of cold sores in babies,” she said.

When she got to the hospital, Henderson’s doctors said she was right to take Brooke’s condition so seriously.

Claire Henderson

Brooke had sores in the back of her throat and on her lips, and was immediately tested to see if she had brain or liver damage.

“If she had been left it could have easily spread and — worst case — been fatal,” Henderson said.