During 1997, an event doctors had been fearing finally occurred. In three geographically separate patients, an often deadly bacterium, Staphylococcus aureus, responded poorly to a once reliable antidote--the antibiotic vancomycin. Fortunately, in those patients, the bacteria remained susceptible to other drugs and was eradicated. But the appearance of S. aureus not readily cleared by vancomycin foreshadows trouble. Worldwide, many strains of S. aureus are already resistant to all antibiotics except vancomycin. Emergence of forms lacking sensitivity to vancomycin signifies that variants untreatable by every known antibio