The deadly H5N1 bird flu virus has spread more aggressively than ever before in wild birds and marine mammals since arriving in South America in 2022, raising the risk of it evolving into a bigger threat to humans, according to interviews with eight scientists.

Of more immediate concern is evidence the disease, once largely confined to bird species, appears to be spreading between mammals. This strain has already killed a handful of dolphins in Chile and Peru, some 50,000 seals and sea lions along the coasts, and at least half a million birds regionwide.

“It’s a matter of time before you will detect the first South American strain in North America,” said Alonzo Alfaro-Nunez, a viral ecologist at University of Copenhagen.

Since the virus was first detected in Colombia in October 2022, there have been two known cases in humans on the continent, one each in Ecuador and Chile. Both came from exposure to infected birds.

While those patients survived, H5N1 bird flu is deadly to humans in roughly 60% of cases worldwide.