CMV is the leading viral cause of congenital birth defects as a result of infection within the womb.
Dr Ian Humphreys said the findings may also have implications for viruses such as flu, hepatitis and even HIV.
Working with colleagues in Oxford, Cambridge and California, researchers at Cardiff University's School of Medicine say they have found that neutrophils - a type of white blood cell - produce a protein that can directly kill virus-infected cells.
The charity CMV Action says in the UK around one baby in 1,000 is born with permanent disabilities including blindness, deafness and brain damage due to the virus.
CMV also affects the immune system of up to half of adults in the UK and is also a major reason behind life-threatening diseases in bone and organ transplant recipients.
Dr Humphreys, of Cardiff University, said it was largely thought neutrophils were specifically designed for killing bacterial infections.
He said: "The fact that they can also fight a viral attack is a major breakthrough.
"Disease may actually be prevented if we can teach the immune system to quickly send antiviral neutrophils to the first site of infection.
"We are now developing a vaccine that may protect the body against CMV."