Dr. Gregg Pane, District of Columbia Health Director, told a radio station late Saturday that biological agent monitors on the National Mall gave positive readings for a small amount of tularemia on September 24 and 25. He urged people who were at the Mall last weekend and who have been experiencing symptoms of pneumonia to immediately seek medical help, but added that there was no evidence that anyone had been affected by the bacteria. Tularemia, which is caused by the bacterium francisella tularensis, can occur naturally and is usually found in animals, especially rodents, rabbits and hares.