The U.S. Centers for Disease Control and Prevention says a California child has become the first U.S. minor to become infected with H5N1 bird flu. The source of the infection is not yet known, but investigators do not believe the child had contact with an infected animal. This follows the weekslong hospitalization of a Canadian teenager in British Columbia, who remains critically ill with an H5N1 infection. Viral genome sequences from the teen revealed mutations that might improve the virus’s ability to spread among human hosts. H5N1 has spread rapidly through U.S. factory farms, mostly affecting poultry before it was discovered in cattle in March. It has since spread to at least 600 herds and has caused infections in dairy and poultry workers. The California Department of Public Health recently detected H5N1 in a sample of raw milk sold in California. President-elect Trump’s nominee for health secretary, Robert F. Kennedy Jr., has promised to lift restrictions on unpasteurized milk and has claimed there are no vaccines that are safe and effective. Infectious disease expert Dr. Michael Mina recently wrote that bird flu could become the next pandemic, warning, “If H5N1 bird flu continues to expand to human transmission … the history books will not look kindly upon the U.S. early efforts to contain it when we absolutely could have take much more aggressive and clear action.”