During swine flu’s first months, the World Health Organization broke down the numbers of new cases by country to show where the virus was spreading. Then, in July, the WHO quietly announced that it was abandoning its count. While this may seem alarming, counting cases has long been counterintuitive. The problem with an exploding epidemic—Britain’s chief medical officer says 100,000 people caught the swine flu in a week—is that the resources it takes to count it grow just as fast. “You can’t devote . . . resources to just surveillance,” says Marc Lipsitch, a Harvard professor of epidemiology. So what can authorities do? Track trends—specifically, how the virus is mutating (U.S. officials recently spotted young swine-flu patients having seizures). By last Friday, the pandemic had spread to nearly every country. From now on, understanding swine flu no longer means asking how many. But instead, what kind?