Claire Henderson from Doncaster uploaded two photos of her daughter Brooke to Facebook showing her mouth riddled with cold sores, with a message to parents.
She wrote: "Please share this with every new mum and pregnant woman you know. Cold sores can be fatal for a baby.
"Before three 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.
"I know this sounds like I am scaremongering, but if my friend had not told me about this my baby girl could have been very seriously ill."
Henderson said as soon as she noticed the cold sores on her daughter she immediately took her to A&E, where she was put on a drip for five days.
Henderson added: "She was VERY lucky, all her tests came back clear.
"The moral of the story is do not let anyone kiss your newborn's mouth, even if they don't look like they have a cold sore and if someone had a cold sore ask them to stay away until it has gone.
"Everyone who I have spoken to had not heard of this before and so I felt it was important to share Brooke's story and raise awareness to stop anyone else going through what we have this week."
The post has been shared more than 31,000 times since it was uploaded on 16 September.
Cold sores are raised and oozing sores or blisters on or close to the lips or inside the mouth, which are caused by a strain of herpes simplex viruses (HSVs).
In most cases, these facial sores are caused by the HSV type 1 (HSV-1) strain.
Herpes simplex viruses can involve the brain and its lining to cause encephalitis and meningitis. In a newborn, herpes viruses can cause severe infections, as well as brain, lung and liver disease or skin and eye sores.
Dr Helen Webberley, dedicated GP for www.oxfordonlinepharmacy.co.uk confirmed what Claire Henderson posted, that it can be extremely dangerous.
She told HuffPost UK Parents: "Herpes can cause cold sores, genital sores, shingles and chicken pox. These are very dangerous to newborns and if anyone has any of these active infections then they should be very careful when handling newborn babies.
"If the virus gets into the blood stream it can go to the brain and cause a fatal type of meningitis."
Webberley explained that sufferers of herpes simplex (cold sores and genital sores) can shed the virus even when they don't have active cold sores.
She added: "It is essential to be vigilant for any signs of a new baby being unwell, look out for a raised temperature, the baby becoming floppy, or off its feed, is the baby quiet, difficult to wake up or generally ‘not right’?
"If you have any concerns seek medical advice as soon as possible.
"It goes without saying that frequent hand washing and hygiene is imperative and, though visitors may not like it, this includes moderating how many people get to kiss the new baby."