Reports of Legionnaires’ disease follow heavy rains.

According to Janet Stout, director of the Special Pathogens Laboratory in Pittsburgh, legionella bacteria is largely resistant to the chlorine that’s used in public water systems to treat water before it is pumped into homes and businesses.

When heavy rains swamp public water systems, more nutrients and sediments get in the water systems, “so more of the water gets past the gates of disinfectants,” she said.