Those peaks coincided with years that saw especially low sea ice formation in the Arctic Ocean. Normally, the ice creates a barrier between the northern reaches of the Atlantic and Pacific, dividing the two bodies of water. But during the two years in question, there were openings large enough for a determined animal—and perhaps their pathogens—to swim through. “It was sort of a perfect storm, because at the end of 2002, around August and September, there was a large outbreak of distemper virus among harbor seals in the North Atlantic,” says Goldstein. September is when sea ice reaches its lowest point for the year, she says.