Journalist Fabiana Frayssinet in Rio de Janeiro and Brian Byrnes in Buenos Aires contributed to this story . Pre-existing conditions contributed to deaths of H1N1 victims , officials say . CDC report summarizes four confirmed cases of U.S. infection . Most hospitalized patients have recovered , report says . U.S. reports 642 cases of H1N1 .
Pre-existing conditions contributed to deaths of H1N1 victims , officials say . CDC report summarizes four confirmed cases of U.S. infection . Most hospitalized patients have recovered , report says . U.S. reports 642 cases of H1N1 .