The server re-checks if you are infected every 5 levels starting with lvl 1, if you get tested and are 'clean' before lvl 6 you can get tested again after lvl 6 (lvl 11, 16, etc)
If you are infected but not been tested the server doesn't bother checking you again (once you have been deemed infected, you will remain infected until you die, even if you don't get tested and giving positive results: midiclorians are a virus and the only known cure is death)