Researchers at the Duke University Medical Center, in Durham, N.C., said that cold sores -- painful, unsightly blemishes around the mouth -- have so far evaded a cure or prevention.

Most of the time, herpes simplex virus 1 lives quietly for years and only produces one molecular product -- latency associated transcript RNA or LAT RNA, Bryan Cullen said.

In studies of mice, the team showed that the LAT RNA is processed into smaller strands, called microRNAs, that block production of the proteins that make the virus turn on active replication. As long as the supply of microRNAs is sufficient, the virus stays dormant.

After a larger stress, however, the virus starts making more messenger RNA than the supply of microRNAs can block, and protein manufacturing begins again. This tips the balance, and the virus ultimately makes proteins that begin active viral replication. The new supply of viruses then travels back to the site of the initial infection at the mouth

The study is published in Nature.