Anthrax was a disease that could be used as a biological weapon.
Some years before Day 3, Marcus Alvers and Michael Amador were suspected of manufacturing weapons grade anthrax. ("Day 3: 1:00am-2:00am")
Washington, D.C. suffered an anthrax outbreak which Michelle Dessler helped to control at some point before she worked for CTU Los Angeles. ("Day 3: 3:00am-4:00am")