Scientists have finally figured out why some people are more prone to cold sores than others.

Cold sores are caused by a strain of the herpes simplex virus, known as herpes simplex virus type 1 (HSV-1). Up to 90% of people are infected with this virus, but only around one in four of these get cold sores frequently.

Until now, it has been unclear why some people seem to be more troubled by the unsightly sores than others.

However Scottish scientists have discovered that people who are more prone have a gene mutation, which stops their immune system from preventing the development of cold sores.

The team from the University of Edinburgh analysed thousands of genes as part of the study. They wanted to identify the genes that expressed the proteins needed by the body's immune system to prevent HSV-1 becoming active.

When they then looked at blood samples from people with cold sores, they found that one of the previously identified genes, known as IL28b, was mutated.

"Most people carry the cold sore strain of the herpes simplex virus, but until now we never knew why only some of them develop cold sores. Knowing that susceptibility to the virus involved relates to people's genes reinforces the need to research, not only the evolution of viruses themselves, but also the susceptibility of hosts to infection," the scientists commented.

Details of these findings are published in the journal, Plos Pathogens.