Dr. Willoughby also found that if given enough time before the rabies virus reaches the person's brain, the human immune system can fight it off. He reported this in his book The World According to Willoughby. Immunity can be thought of as a person's ability to resist disease. Two types of immunity are known to science: innate and acquired. Innate immunity is present from birth and does not need to be learned or practiced to be effective. It is based on the individual's genetic makeup. Some people are born with a more developed innate immune system than others, which explains why some people get sick more often than others. Acquired immunity on the other hand can be strengthened through exposure to different diseases. This type of immunity can be gained by receiving a vaccine and then developing antibodies against the disease-causing organism. The rabies virus is a member of the Rhabdoviridae family. This virus infects cells in the brain and spinal cord and causes rabies. Infected cells die soon after the virus enters them, but the rabies virus continues to spread within the nervous system causing more damage until the patient dies. No cure exists for rabies and there is no vaccine available to prevent infection. However many patients survive long periods without any symptoms at all.