AFRICA HAS reported some genuinely good news in the battle to eradicate polio. Late last month , Nigeria passed a full year without a case of wild poliovirus. As of Aug. 11, it has been a year since the last case was detected anywhere on the continent (it was in Somalia). These anniversaries are unofficial milestones, but they point toward continued progress against polio, a scourge that once claimed hundreds of thousands of lives each year. Unfortunately, polio has shown a fierce tendency to return. Hopefully this time will be different.