Some people develop multiple sclerosis after an infection. Could a vaccine prevent that — and what does it reveal about the long-term effects of viruses?

In a sprawling facility in Silver Spring, Maryland, the US Department of Defense (DoD) has amassed a hoard of epidemiological treasure. Walk-in freezers each the size of a basketball court hold 72 million vials of blood serum meticulously tracked and sorted into cardboard boxes stacked nearly 4 metres high. Technicians pull on winter coats and gloves for 20-minute trips into these −30 °C deep freezers. The vials they bring out hold untold riches.

For Alberto Ascherio, an epidemiologist at Harvard T.H. Chan Medical School in Boston, Massachusetts, the vials have yielded a rare gift in the quest to discover the cause of multiple sclerosis (MS), a disease in which the immune system attacks nerve cells.Researchers have long suspected a link between MS and the Epstein–Barr virus (EBV), but it has been hard to establish a strong connection, partly because almost everyone gets an EBV infection at some point, most of them harmless. The samples in the DoD’s freezers provided an unparalleled chance to explore the link. After analysing data and samples collected from more than 10 million army, navy and air force service members since 1993, Ascherio found that EBV infection increases the risk of MS 32-fold¹.

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