A headline in the Washington Post blares:
Japan's New Public Health Problem Is Getting Big
Obesity Has Grown, Along With Appetite For Western Foods
But no. Obesity is not a public health problem. It is apparently becoming more widespread in Japan, though still much less so than in the United States, but it remains an individual and non-contagious problem.
The meaning of "public health" has sprawled out lazily over the decades. Once, it referred to the project of securing health benefits that were public: clean water, improved sanitation, and the control of epidemics through treatment, quarantine, and immunization. Public health officials worked to drain swamps that might breed mosquitoes and thus spread malaria. They strove to ensure that water supplies were not contaminated with cholera, typhoid, or other diseases. The U.S. Public Health Service began as the Marine Hospital Service, and one of its primary functions was ensuring that sailors didn't expose domestic populations to new and virulent illnesses from overseas.
Those were legitimate public health issues because they involved consumption of a collective good (air or water) and/or the communication of disease to parties who had not consented to put themselves at risk. It is difficult for individuals to protect themselves against illnesses found in air, water, or food. A breeding ground for disease-carrying insects poses a risk to entire communities.
The recent concern over a tuberculosis patient on an airplane raises public-health issues. You might unknowingly find yourself in an enclosed space with a TB carrier. But nobody accidentally ingests a Big Mac. And your Big Mac doesn't make me fat. That's why obesity is not a public health issue, even if it's a widespread health problem. As I wrote before,
Language matters. Calling something a "public health problem" suggests that it is different from a personal health problem in ways that demand collective action. And while it doesn't strictly follow, either in principle or historically, that "collective action" must be state action, that distinction is easily elided in the face of a "public health crisis." If smoking and obesity are called public health problems, then it seems that we need a public health bureaucracy to solve them -- and the Public Health Service and all its sister agencies don't get to close up shop with the satisfaction of a job well done. So let's start using honest language: Smoking and obesity are health problems. In fact, they are widespread health problems. But they are not public health problems.