CHICAGO—U.S. Rep. Jesse Jackson Jr., who defeated a primary challenge this year despite being the target of a House ethics investigation, has been on a medical leave for two weeks and is being treated for exhaustion, his office announced Monday. In a three-sentence news release, Jackson's office disclosed that the Democrat went on leave June 10 but did not provide further details, including how long he would be away. In the release, he asked that his family's privacy be respected.