Multiple sclerosis (MS) may be linked to the combined effects of a viral infection and not getting enough sun, research suggests.
A study found that, together, the two factors accounted for 72% of variations in MS occurrence across the UK.
Levels of sunlight exposure alone explained 61% of the difference between high and low rates of MS.
Previously, it was known that people with a history of glandular fever, a common infectious illness caused by the Epstein-Barr virus, were more at risk of MS.
A pattern of higher incidence had also been seen in individuals whose skin is exposed to little sunlight.
"We wanted to see whether the two together would help explain the variance in the disease across the United Kingdom," said lead researcher Dr George Ebers, of Oxford University.
MS is an autoimmune disease which destroys myelin, the fatty insulating sheath that surrounds nerve fibres.
Loss of myelin leads to the disruption of nerve signals and symptoms ranging from mild tingling and numbness to paralysis.
Around 100,000 people in the UK suffer from the condition.
Research has shown that MS is more common at higher latitudes away from the equator, where there is less exposure to ultraviolet rays from the sun.
The Oxford scientists, whose findings were reported in the journal Neurology yesterday, looked at all admissions to NHS hospitals in England over a period of seven years.
They identified 56,681 cases of MS and 14,621 cases of glandular fever, also known as infectious mononucleosis.
Levels of ultraviolet sunlight intensity in England were also analysed using satellite data from American space agency Nasa.
Vitamin D deficiency caused by not getting enough sun may be behind the link, the scientists believe.
"It's possible that vitamin D deficiency may lead to an abnormal response to the Epstein-Barr virus," said Dr Ebers.
He added: "Lower levels of UVB (ultraviolet B rays) in the spring season correspond with peak risk of MS by birth month.
"More research should be done on whether increasing UVB exposure or using vitamin D supplements and possible treatments or vaccines for the Epstein-Barr virus could lead to fewer cases of MS."
The research was funded by the MS Society, the Medical Research Council and the Wellcome Trust.
Dr Doug Brown, head of biomedical research at the MS Society, said: "This work adds weight to existing evidence that MS is caused by a number of factors working in combination.
"Vitamin D has been closely studied in recent years and is thought to be a key factor in the development of MS; we look forward to seeing more research dedicated to this important area."