WASHINGTON D.C. (WTVB) - U.S.health officials are warning about a whooping cough epidemic.  The Centers for Disease Control and Prevention says current rates of whooping cough are approaching a 50-year high.  So far this year, the CDC says 18-thousand cases of the highly contagious disease have been reported.  Nine infants have died this year from it.  Infants are especially susceptible to the disease.  It causes uncontrollable coughing and breathing difficulty.  More than three-thousand cases of whooping cough have been reported inWashingtonstate this year, more than three times the total cases for all of 2011. The CDC is investigating whether the rise in whooping cough cases may be due to the vaccine for it wearing off in older children.  The state of Washington has purchased thousands of doses of whooping cough vaccine and distributed them in needy communities.