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

Brooke Rollins, the president’s nominee to lead the Agriculture Department, appeared before the Senate Agriculture Committee on Thursday.

The cases have spurred new pet food safety rules and poultry surveillance efforts.

The administration is committing an additional $306 million toward battling the virus, and will distribute the money before President-elect Donald J. Trump takes office.

While some agree with Robert F. Kennedy Jr. that the agency needs reform, their ideas for fixing it are very different from his.