Short version: 18-19/20 people end up being infected with the Epstein-Barr virus at some point in their life. It causes mono, among other things. Half of all kids get it as soon as their immune system starts working on their own and typically it causes mild symptoms at worst. Complications to EBV cause some people to develop the auto-immune response known as Multiple Sclerosis.
mRNA technology might be effective at providing a vaccine for EBV, which would solve or lessen the severity of a wide swath of future problems. This is mRNA HIV vaccine trials big (potentially)