When the time came, just a handful of reporters were on the line. Also there: the CDC’s top flu experts. They sounded worried.

Two unrelated children from southern California had tested positive for flu infections caused by viruses that normally sicken pigs. This could have been a fluke, but from the unusual nature of the call and from the tension in the voices of the experts, it was clear that the CDC officials thought it might be something else: an influenza pandemic.