It points out that in 1978, after smallpox was thought to have been eradicated, a photographer working at Birmingham Medical School came down with and died of the disease. A lab there, it adds, was studying smallpox and it managed to get out, likely due to mismanagement. More recently, it notes that a vial of smallpox was uncovered in 2014 in an old Food and Drug Administration storage room where it may have sat since the 1950s.
According to Vox, there are nearly 300 labs in the US that are approved to work with select agents like plague or anthrax, and between 2005 and 2012, the Centers for Disease Control and Prevention's Select Agents and Toxins program received 1,059 reports of releases.
Most of these recent releases, Vox adds, are due to human error. This, Harvard's Marc Lipsitch tells Vox suggests that the there will never be no risk of release, and that usually is a good balance between safety and research. He adds, though, that for some dangerous pathogens, the risk might not be worth the reward.