Is it true cold sores can be fatal to infants?

Not likely: Most children are exposed the virus that causes herpetic lesions (cold sores) in early childhood without symptoms. A few will have a mild fever. Some will develop a very sore mouth and eating is painful. This is very rare, passes on its own without treatment but there is a danger and infant could become seriously dehydrated and need hospitalization. This reaction happens only on the first exposure.

Yes: Cold sores are usually fatal up to four weeks of age.

Yes: Herpetic infections of the newborn and early can be fatal because of the possibility of the infection spreading to the brain.Although rare it can happen.