Most diseases that affect dogs or humans are not transferable between the two species. There are some exceptions:
Rabies
Rabies or Hydrophobia (name derived from the fact that infected individuals usually show an aversion (phobia) to water hydro)), is a usually fatal disease which can be transmitted to dogs or humans by the bite of many mammals, including dogs, cats, raccoons, and bats. 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 which would infect them.
In the first stage of the disease the infected dog will exhibit change in behavior and fear. In a second stage the animal's behavior becomes more and more aggressive, increasing the chances that they will bite another animal or a person and transmit the disease. In the third and last stage rabies-infected animals suffer deterioration of the brain leading to loss of coordination and trouble with bodily functions.
Areas which are rabies-free, (usually islands) such as Britain, Australia or the U.S. state of Hawaii, have strict quarantine laws to keep their territories rabies-free requiring long periods of isolation and observation of the animal, which make it very unattractive in practice to move there with a pet unless it is quite young. Areas which are not rabies-free usually require that dogs (and often cats) be vaccinated against rabies. A person or dog bitten by an unknown or unvaccinated dog (or other animal) should always be treated without waiting for symptoms, given the potentially fatal consequences if the biter was rabid; there has only been one case in history of someone surviving rabies when treatment was not begun until after symptoms appeared. The biter should be apprehended if at all possible, as only euthanasia and autopsy of the brain can determine if it was rabid. This should be a great incentive to dogowners 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, as well as the need for it to be treated for rabies if it is suspected of being bitten.
Parasites
Parasites, particularly intestinal worms such as hookworms, tapeworms and roundworms, can be transmitted in a dog's saliva or feces (see our article Internal parasites). Some have fleas as intermediate hosts; the worm egg must be consumed by a flea to hatch, then the infected flea must be ingested (usually by the dog while grooming itself, but occasionally by a human through various means) for the adult worm to establish itself in the intestines. The worm's eggs than pass through the intestines and adhere to the nether regions of the dog, and the cycle begins again.
Fleas and ticks of various species can be acquired and brought home by the dog where they can multiply and attack humans (as well as vice versa). This is particularly important, now that tick-borne Lyme Disease has become endemic throughout a large area, in addition to other similar diseases such as Rocky Mountain Spotted Fever. Although dogs do not seem to be as susceptible to such diseases as humans, similar rickettsial diseases have been spread by dogs to humans through such mechanisms as a dog killing an infected rabbit, then shaking itself off in the house near enough to its owners to fatally infect most of the family.
Leptospirosis
Leptospirosis is a severe bacterial disease that affects the internal organs of humans and animals. It is caused by bacteria of the genus Leptospira that is often carried by rodents. Humans and dogs become infected through contact with water, food, or soil containing urine from infected animals. The bacterium enters through mucous membranes and spreads quickly throughout the body. This may happen by swallowing contaminated food or water or through skin contact, especially with mucosal surfaces, such as the eyes or nose, or with broken skin.
