Every time you brush your teeth, take a breath, or eat a meal, you expose your body to microbes. Countless more tiny lives reside full time on your skin and in your guts; they’re a normal part of the human flora. Yet the ubiquity of such foreign organisms poses a big problem for the immune system.

"Against this background of harmless microbes we’re constantly interacting with, the immune system needs to know it has encountered something more dangerous," says Russell Vance. A Berkeley professor of immunology and pathogenesis, Vance studies how cells sense and attack intracellular invaders. “Pathogenic microbes do things that distinguish them from harmless microbes. We are trying to prove that those actions provoke specific immune responses," Vance says.

Vance’s subject of choice is the bacterium Legionella pneumophila, the infectious agent behind Legionnaire’s disease. It was first identified in 1976 after killing 34 people and sickening more than 200 others in a hotel hosting an American Legion meeting. The microbe normally lives inside freshwater amoebae; in this case, it infected the hotel’s air cooling system.

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