Multiple sclerosis (MS) is almost always a delayed response to infection with the Epstein-Barr virus, a study of 10 million former military personnel in the US indicates. The findings could provide clues on how to treat the disease - which is hallmarked by the immune system attacking the myelin sheath that protects the brain and spinal nerves - and raise the urgency of preventing the common virus in the first place. More broadly, it serves as a reminder that even when people appear asymptomatic or recover quickly from viral infections, there can be serious consequences down the track.

The Epstein-Barr virus is part of the herpes family of double-stranded DNA viruses. Its high transmissibility through kissing, spitting, or sharing food means the overwhelming majority of the population has been infected by their late 20s. Its most common effect is infectious mononucleosis, better known as glandular fever, which can leave people feeling exhausted for weeks and, in rarer cases damage the liver or spleen.

Epstein-Barr has also been linked to much rarer, but far more serious, conditions such as certain blood cancers. For decades medical researchers have suspected it may be a cause of MS, but have struggled to prove or disprove the theory. Access to the serum samples taken from more than 10 million Americans serving in the military has led to publication in Science of powerful evidence supporting the idea.