With about 55 million U.S. children heading back to school in the next few weeks, concerns are growing that the H1N1 swine flu will spread even further than it already has. Identified by scientists four months ago, the virus has already turned up in nearly every corner of the world, from Argentina to Iran. It defied public-health officials' predictions of a lull in the warm summer months, proliferating in military units and children's summer camps.

More than two million people are believed to have contracted the new flu in...