Rabies is a contagious viral disease transmitted from the bite of an infected animal. Animals that are known to pass the disease to humans are dogs, cats, bats, rats and raccoons. The virus is found within the saliva of an infected animal and any bite that breaks the skin will result in infection as the virus, although very slow to act, is very virulent within a host. However, the disease can only be transmitted in this way and casual transmission is impossible. Animal corpse handlers in India have been known to become infected from handling infected carcasses, and cave explorers have been infected by passing through caves whose air had a great concentration of bat saliva droplets.
Rabies slowly attacks nerve and brain cells, gradually destroying them and leading to seizures, dementia, paranoia aggression and insensitivity to pain. Ironically, this makes animals with the disease far more aggressive than normal, and less likely to back down from a confrontation with other animals, making it more likely that they will pass the infection to other animals, and to humans.
One clear sign of infection is hydrophobia - the body's tissues start to reject fluids, resulting in the "foaming at the mouth" that is a key indicator of the disease.
Vaccination against rabies is fairly straightforward as injection with dead virus will build the antibodies the body will need in the event a live infection arises. In addition, patients can be successfully vaccinated after infection as the dead virus will give rise to antibodies far more quickly than the live virus will. However, once the virus is far enough along to cause symptoms in an infected person, the disease is untreatable and will eventually result in the patient's death.