More than half a century ago, when antibiotics were transforming modern medicine, a now almost forgotten drug was hailed as something close to the miracle of miracles. Doctors rushed to prescribe it for an array of medical problems — that is, until they discovered that the drug, chloramphenicol, sometimes had lethal side effects. Yet today, improbable as it may seem, an effort is underway to revive the use of chloramphenicol and other antibiotics that had largely been banished because of their potential danger. How can this be? Some scientists say the older antibiotics may be one way to fight sometimes deadly bacteria that have become resistant to modern drugs.