Baby Kaiden McCormick fell ill after catching a cold sore virus from Carl Maclaren, 24.
The baby spent six weeks on a life-support machine, but died aged only two months.
The cause of death was multiple organ failure, triggered by the herpes simplex virus.
His mother told The Sun: "When they said it was herpes, I just couldn't believe it. I asked how it was possible for a baby to have herpes.
"When they said it was transmitted via the cold sore virus I looked at Carl and he instinctively touched his lip.
"He is totally distraught and kept saying he blamed himself. All he'd ever done was shower our baby with love and affection."
Kaiden was born five weeks premature on 28 March 2012 and was allowed home after three days.
However, the baby was rushed to hospital and died after suffering seizures.
Doctors informed police that the illness was unexplained, but no charges were brought after the death was established as a tragic, unforeseen accident.
The couple are now calling on the government to ensure that all expectant parents are aware of the dangers of cold sores.
"Nobody told us that Carl's cold sore posed such a risk," said McCormick.