Ultraviolet rays could be used to combat the spread of chickenpox, according to researchers at the University of London.
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.
"In the dry season, the pollution in the atmosphere reflects the UV rays back into space before they reach us. But in monsoon season, the rains wash away the pollution, meaning the UV rays can get through."
He said findings also show why two distinct genetic types of the virus have formed - a temperate type and a tropical one.
He found the temperate type only transmitted in the tropics when UV radiation was either reduced or negated, for example in the home but not outside.
For the tropical type, it transmitted whe UV rays were present, suggesting it had some resistance to sunlight.
"For the temperate virus line to have lost the selective advantage of resistance to UV rays as it broke off from the original tropical virus, it must have gained an advantage in the virus life cycle as an evolutionary trade-off," Dr Rice added.
"An obvious advantage would be an ability to reactivate more easily, as shingles. The virus can only have one of these survival advantages, not both.
"This might explain why shingles appears to be so much less common in people from the tropics, and why the temperate virus reactivates much more readily than the tropical type."
He said his findings could help the development of new treatments for chickenpox and shingles, and said more studies are needed to fully examine the effect of UV rays on the virus.