It is not often clear when the spread of a disease, such as cholera, becomes a pandemic.
After decades of warnings about the inevitability of another pandemic of influenza, it is astonishing that health officials have failed to make clear to the public, even to many colleagues, what they mean by the word pandemic.
Generations of people have used the term to describe widespread epidemics of influenza, cholera and other diseases. But as the new H1N1 swine influenza virus spreads from continent to continent, it is clear that a useful definition is far more complicated and elusive than officials had thought.
And what is at stake is far more than an exercise in semantics. A clear understanding of the term is central to the World Health Organizationís six-level staging system for declaring a pandemic, which in turn informs countries when to set their control efforts in motion.