When an animal is infected with the rabies virus, the virus multiplies within the body until it results in symptoms. The period between infection and the onset of symptoms is known as the incubation period. The rabies incubation period may vary from a few days to several years, but typically lasts one to three months.
When the rabies virus reaches the brain, it multiplies quickly and rabies symptoms begin. Rabid animals may be aggressive and vicious, or lethargic and weak. In people, early rabies symptoms of fever, headache, and fatigue are usually followed by confusion, agitation, hallucination, and paralysis. Once symptoms of rabies begin, the disease is almost always fatal.