A pandemic is not inevitable, scientists say. But the outbreak has passed worrisome milestones in recent weeks, including cattle that may have been reinfected.

Bird flu cases are still rising in the U.S. as the virus continues to devastate poultry farms. More than 145 million chickens, ducks, turkeys and other fowl have been slaughtered across the United States since the outbreak began in January 2022. As for humans, 67 people have been infected, according to the Centers for Disease Control.

The United States has reported its first outbreak of H5N9 bird flu in poultry on a duck farm in California, the World Organisation for Animal Health (WOAH) said on Monday.