Scientists say this is the first time marine mammals have been found to carry the H1N1 flu strain, which originated in pigs. The seals seem to have picked up the virus while at sea, but it’s unclear how this happened.

“We thought we might find influenza viruses, which have been found before in marine mammals, but we did not expect to find pandemic H1N1,” Tracey Goldstein, an associateprofessor with the UC Davis One Health Institute and Wildlife Health Center, said in a statement. [10 Deadly Diseases That Hopped Across Species]

H1N1 was circulating in humans in 2009,” Goldstein added. “The seals on land in early 2010 tested negative before they went to sea, but when they returned from sea in spring 2010, they tested positive. So the question is, where did it come from?”

Contact with humans carrying the virus is unlikely when the elephant seals are at sea, because the creatures spend most of their time looking for food in a remote part of the northeast Pacific Ocean off the continental shelf.