''Toxic'' is apparently more than just the title of Britney Spears' recent single. The singer has canceled her third concert in three weeks for health reasons, postponing Thursday night's Cleveland show even as fans were lining up in the rain at the Gund Arena. Publicist Leslie Sloane Zelnik told the Associated Press she didn't know the specifics of Spears' ailment, but Cleveland TV station WKYC quoted a spokesperson as saying Spears had come down with the flu. The show, postponed until June 29, follows recently scrapped dates outside Chicago and Detroit, with Spears citing a knee injury she suffered during a dance move onstage. Note to Britney: If you're seriously planning to tour China, as recent reports have suggested, be sure to get your shots.