Tularemia is a rare but highly infectious disease that resides in small mammals such as rodents and rabbits, and arthropods, such as ticks. The disease is caused by the bacterium Francisella tularensis. Tularemia is most often transmitted to humans by tick or biting fly bite, handling of an infected animal, or inhalation or ingestion of the bacterium. People have not been known to transmit the infection to others. The severity of tularemia varies greatly. Some cases are mild and self-limiting; others may have serious complications, and a small percentage (about 2 percent) are fatal.