The affected people have reported flu-like symptoms, including fever, muscle fatigue, nausea and respiratory distress, the university said in a release.
No Duke student-athletes were exposed to the bacteria or have reported illness, the university said.
Legionella is not contagious.
"Duke infectious disease specialists have worked with federal, state and local public health officials to identify the source of the exposure, which likely occurred in a training room in the Schwartz-Butters Building," the release said. "All the individuals who were exposed to the bacteria have been positively identified and have been contacted by Duke physicians."
All the people who became ill are expected to fully recover, Duke said.
The exposure appears to have been limited to the training room, which has been temporarily closed.
Duke said there is no continuing risk to employees or visitors.