Narrator: This is Science Today. Tuberculosis is a major killer in developing countries around the world. But epidemiologist Sally Blower of the University of California, San Francisco says the story is different in Europe and North America.

Blower: The mortality from TB has been declining dramatically for about a hundred years, and there wasn't effective treatment, drugs against TB until about the 1940s and 1950s. So a lot of this decline was long before there were widespread drugs.

Narrator: Some of the decline is due to improved sanitation and standards of living. But according to a mathematical model created by Blower and her fellow researchers, there's another reason that's out of human control.

Blower: And sort of the simplest and I think most interesting result is that TB epidemics go up and down on a cycle of about a hundred years or longer. They're very very slow epidemics.

Narrator: In other words, TB should be declining anyway, because we're on the tail end of a hundred-year epidemic. Although another epidemic seems to be getting underway. For Science Today, I'm Steve Tokar.