US government infectious disease labs mishandled dangerous pathogens five times in the last decade, according to a health agency report.
This year alone, workers mishandled samples of anthrax and the highly-infectious H5N1 avian flu.
In response, the Centers for Disease Control and Prevention (CDC) has closed the two labs involved.
The agency has also temporarily barred high-security labs from transporting dangerous pathogens.
There have been no reported infections from previous cases, and no-one potentially exposed to anthrax has shown signs of illness, CDC officials said.
“These events should never have happened,” CDC Director Tom Frieden told reporters on Friday.