Lyme disease is transmitted to humans by the bites of infected ticks of the genus "Ixodes". In the United States, ticks of concern are usually of the "Ixodes scapularis" type, and must be attached for at least 36 hours before the bacteria can spread. In Europe, ticks of the "Ixodes ricinus" type may spread the bacteria more quickly. In North America, the bacteria "Borrelia burgdorferi" and "Borrelia mayonii" cause Lyme disease. In Europe and Asia, "Borrelia afzelii" and "Borrelia garinii" are also causes of the disease. The disease does not appear to be transmissible between people, by other animals, or through food. Diagnosis is based upon a combination of symptoms, history of tick exposure, and possibly testing for specific antibodies in the blood. Blood tests are often negative in the early stages of the disease. Testing of individual ticks is not typically useful.