RENEE MONTAGNE, host: Health officials use different terms to describe the emergence of a particular illness, and the progression from outbreak to epidemic to pandemic. And so, let's take a moment to define exactly what those words mean.</s>RENEE MONTAGNE, host: An outbreak is a sudden increase in the numbers of a certain illness. That's about where we are now and have been for the past several weeks with the swine flu. What caused doctors in Mexico to sit up and take notice was a sudden jump in serious pneumonias at the end of the regular flu season. Tests are underway now to determine how many of these cases might be swine flu.</s>RENEE MONTAGNE, host: An epidemic - that's an outbreak of a contagious disease that spreads rapidly and widely. Since officials aren't sure how many of the cases of pneumonia in Mexico are actually swine flu, it's still a little early to call it an epidemic. And a few cases in the U.S. and Canada are definitely not an epidemic, at least not yet.</s>RENEE MONTAGNE, host: A pandemic is where a disease has spread across a wide geographic area and is affecting a large portion of the population, typically millions. We're not there yet with swine flu. We're not even near there yet with swine flu. But public health officials worry we might get there if the outbreak in Mexico, the U.S., and Canada grows larger and spreads to other countries.</s>RENEE MONTAGNE, host: You're listening to MORNING EDITION from NPR News.
Health officials use different terms to describe the emergence of a particular illness, and the progression from outbreak to epidemic to pandemic.