Wastewater in several Californian cities, including San Francisco and Los Angeles, recently tested positive for bird flu. But understanding disease risk and exposure to humans isn’t so straightforward

There have been dozens of confirmed human cases of bird flu in the U.S. this year. Experts discuss the symptoms, risks and how to prevent getting sick.

Another cause for concern: flu season is underway in the U.S., and as seasonal influenza virus goes around, “humans, ourselves, could be a mixing vessel,” Davis says. If a person were simultaneously infected with bird flu and seasonal flu, the two viruses could theoretically combine to create a more transmissible strain.

Canada detected its first human case of the new strain. Sen. Elizabeth Warren (D-Mass.) said Monday that President elect-Trump “and his transition team are already breaking” a law on presidential transition.