Rabies, or Hydrophobia, is an almost invariably fatal disease caused by a virus, which can be transmitted to dogs or humans by the bite of an infected mammal. Although rodents and similar small mammals can be infected with the disease artificially, they are generally not found infected in the wild; the current hypothesis is that they are not likely to survive any attack that would infect them.

Animals with rabies suffer deterioration of the brain and tend to behave bizarrely and often aggressively, increasing the chances that they will bite another animal or a person and transmit the disease. Areas that are rabies-free, (usually islands) Australia, and the American state of Hawaii have strict quarantine laws to keep their territories rabies-free. These require long periods of isolation and observation of imported animals, which makes them unattractive places to move with a pet. Britain and Ireland are also rabies free, but they recently introduced chipping and vacination schemes to make it easier for dog, cat and ferret owners.

Areas that are not rabies-free usually require that dogs (and often cats) be vaccinated against rabies. In Europe the virus is mainly carried by the fox and it is the law in many northern European countries for pets to be vaccinated against it. A person or dog bitten by an unknown dog (or other animal) should always be treated without waiting for symptoms, given the potentially fatal consequences of a rabid biter: there has been only one case of someone surviving rabies when treatment was not begun until after symptoms appeared. The biter should be apprehended if possible, as only autopsy of the brain can determine if it was rabid. This should be a great incentive to dog-owners to vaccinate their dogs even if they feel the risk of their dog contracting rabies is low, since vaccination will eliminate the need for their dog to be euthanized and examined in this fashion should it bite anyone or be suspected of biting anyone. This applies to dogs that are showing neurological signs at the time of the bite.

Unvaccinated healthy dogs need to be confined for ten days from the time of the bite (at home or at a veterinarian depending on state law). If the dog is not showing signs of rabies at the end of ten days, then the bitten person could not have been exposed to rabies. Dogs and cats do not have the rabies virus in their saliva until a few days prior to showing symptoms. Ten day confinement does not apply to any other species. A dog or cat bitten by a wild animal in an area known to have rabies should be confined for six months, because it can take that long for symptoms to show.