U.S. Health and Human Services Secretary Robert F. Kennedy Jr. is appearing before a congressional committee Thursday, where he’s facing tough questions about the turmoil at federal health agencies he oversees. Kennedy was asked early on in the hearing about the recemt firing of Susan Monarez, a longtime government scientist he installed as the CDC director for less than a month. Kennedy accused Monarez of lying in her Wall Street Journal op-ed on Thursday when she said she was pressured to preapprove the recommendations of his vaccine advisory panel. He also defended her firing along with others who have left the CDC, telling lawmakers "they did not do their job." “We are the sickest country in the world,” he said. “That’s why we have to fire people at the CDC. They did not do their job.” Kennedy got into a heated exchange with Oregon Senator Ron Wyden, a Democrat, who accused the secretary of replacing scientists with "vaccine skeptics and conspiracy theorists. Kennedy said his actions "got rid of the conflicts of interests" he said had pervaded such panels. Kennedy said he had support from scientists and doctors "all over the country." Many of the nation’s leading public health and medical societies, including the American Medical Association, American Public Health Association and the American Academy of Pediatrics have decried Kennedy’s policies and warn they will drive up rates of vaccine-preventable diseases.