Francisella tularensis is an unusual, gramnegative, rod-shaped bacteria that causes the disease tularemia. Francisella tularensis is unusual because of its virulence, the geographic distribution of the few known varieties, and the organism's unique preference for residing inside macrophages and neutrophils (it appears to prevent the oxidative burst and can escape the phagosome to persist in the neutrophil). Tularemia is acquired in most patients through contact with infected tissues of cottontail or jackrabbits or from bites of dogs, cats, snakes, and other animals that have become contaminated from biting or eating infected rabbits. Human infections can be induced by ticks, mosquitoes, and deer flies that have fed on an infected animal.

Tularemia has an American heritage. McCoy studied a plaguelike disease in ground squirrels in 1911 in Tulare County, California. Dr. Edward Francis subsequently selected the name tularemia (1921) for the disease because his investigations demonstrated that a bacteremia must occur and because of the first demonstration of infected animals in Tulare County. The organism was subsequently named Francisella tularensis to honor the extensive studies of Dr. Francis. Further studies have identified infection in more than 100 wild and domestic animals and the ectoparasites and biting insects that feed on them.