Scientists collect organic material from a dead porpoise on the coast of the Atlantic Ocean, during a bird flu outbreak in São José do Norte, Brazil.Diego Vara/ReutersThe H5N1 bird flu virus is spreading through US cattle herds for the first time.The mammal-to-mammal transmission has scientists worried the virus could mutate to spread between humans.Former surgeon general Jerome Adams fears the US is making the mistakes of 2020 all over again.Bird flu is flying wild.