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.