The exposures would have occurred since August 9th, but the hospital says no patients have been identified who have developed the illness. The hospital says it has been contacting families of those who may have been exposed.

The three infected staffers -- a respiratory therapist and two nurses -- all work in the pediatric intensive care unit.

Doctors say they are dealing with an outbreak.

Whooping cough is a contagious bacterial infection that usually causes prolonged coughing. It can infect people of any age. Children's Hospital says the incubation period is usually seven to ten days but may be as long as 21 days. It can be fatal in some cases.

"Any patient that's in our hospital that could come in contact with this germ could be effected more adversely. We want to take all safety precautions to prevent the spread to them," said Dr. Craig Rubens, the Head of Infectious Diseases at the hospital.

Children's Hospital implemented new precautions Friday night to contain the outbreak. Workers in the PICU are now wearing masks over their faces and only the parents of children are being allowed into the ICU to visit.

Doctors say they won't know if they have the outbreak completely under control for at least a month.