Ultraviolet rays could help prevent the spread of chickenpox, new research has shown.
The discovery, which suggests people in milder climates are more at risk of catching the disease, is hoped to lead to new ways of preventing chickenpox and its more severe relative, shingles.
Dr Phil Rice, virologist at St George's, University of London, found chickenpox is much less common in places with high UV ray levels.
UV light is known to inactivate some viruses, and Dr Rice believes his findings show UV rays could inactivate the varicella-zoster virus - responsible for chickenpox and shingles - on the skin before it transmits to another person.
His discovery explains why there is less transmission in the tropics, where chickenpox is much less frequent than in temperate countries.
It also explains why chickenpox peaks in temperate zones - where it is seasonal - in winter and spring, when UV rays are lowest.
Dr Rice examined data from 25 studies on varicella-zoster virus prevalence patterns in temperate and tropical areas across the globe. He plotted the data against a range of climatic factors, to look at what might be the most likely causes of increased prevalence.
The data showed that, once other factors were ruled out, UV rays were the only factor to match the patterns in each country studied.
Dr Rice, whose study has been published in Virology Journal, said: "No one had considered UV as a factor before, but when I looked at the epidemiological studies they showed a good correlation between global latitude and the presence of the virus."
He said one convincing factor for the hypothesis was that there was an explanation for each anomaly: "For example, the peak incidence of chickenpox in India and Sri Lanka is during the hot, dry, sunny season. You would expect chickenpox to be at its lowest at this time, so at first this didn't fit the theory. However, this was explained because UV rays are actually much lower in the dry season compared with the monsoon period."