Rabies is an infection caused by a family of viruses known as Rhabdoviridae that affect humans and many animals. In the United States rabies is seen mainly in bats, foxes, raccoons, and skunks. In developing countries, most transmission, especially to people, is from dogs. The virus (there are at least eleven strains, including the "classic" virus seen most often) is found in an infected animal's saliva, and transmitted to another animal -- or a person -- by biting or by licking an open wound. Most patients with rabies in developed countries acquire their virus from wild animals -- in the United States the most common sources are bats. There was one documented case in the United States in 2004 of four people developing rabies after receiving transplants from a donor who died of encephalitis which, on further evaluation, turned out to have been due to rabies and who had been bitten by a bat. There have also been cases of patients dying of rabies after reciving cornea transplants from infected donors.
The rabies viruses primarily infect neurons (nerve cells). Once the virus enters a wound, it travels slowly up nerve fibres in the area of the wound until it reaches the spinal cord; this process is the "incubation period" and can last from a few days to one year in humans. Typically, it takes 1-3 months after the initial infection for the virus to reach the spinal cord. From there it spreads quickly to the brain; on arrival in the brain, the virus reproduces much more rapidly and spreads back down other nerves as well; the salivary glands contain many neurons, so the virus is produced there and is shed in the saliva.
The incubation period is the first of 5 stages to a rabies infection. The second stage is similar to the flu, with low-grade fever, headache, sore throat, loss of appetite, nausea, vomiting, irritability, and malaise (feeling lousy). There may also be pain, itching, and/or numbness where the patient was bitten. This stage, called the prodrome, last up to a week.
The third stage is the acute stage. This lasts for up to 1 week, and usually takes one of three forms.
The fourth stage is coma and general paralysis with circulatory and respiratory failure. This usually lasts at most 2 weeks, at the end of which the patient dies (the fifth stage).
Only one person has ever survived a rabies infection: a 15-year-old girl bitten by a bat 1 month before developing symptoms. She was in the hospital for 2-1/2 months and on a mechanical ventilator for almost 1 month, and received ribavirin (an antiviral antibiotic used in the past to treat RSV). She was also kept in a medication-induced coma to reduce the risk of brain injury. Although she went home, she had many neurologic problems which persisted after her discharge. She may have been helped by the treatment, but she had several problems related to therapy, and she may have been infected with a strain of the rabies virus which wasn't as damaging as other strains. And the treatment itself -- especially the ribavirin -- had numerous side effects.
Obviously, it's better not to be exposed to rabies in the first place, or to be vaccinated. Since the virus travels very slowly to the spinal cord, a full course of vaccine and of rabies immune globulin (RIG) will almost always provide enough passive and active immunity for the body to dispose of the virus before it reaches the brain. You must receive the full course for complete protection; the vaccine is given in 5 doses, with the last 4 doses given 3 days, 7 days, 14 days, and 28 days after the first dose.
Very few people need routine vaccination against rabies. These include veterinarians and veterinary workers, and lab technicians. People who explore caves for fun or profit should also be vaccinated since they may be bitten by bats. Travellers to countries where large numbers of dogs are infected might want to be vaccinated as well, especially if they will be in remote areas where they might not be treated immediately after a bite. In the developed world, the main method of controlling rabies is to vaccinate all house pets, including dogs and cats but also other pet animals such as ferrets. It is possible for an animal to be infected with rabies despite being vaccinated; because of this if you are bitten the animal is observed for 10 days (about the time it takes a dog or cat to develop full-blown rabies) and you are treated only if the animal shows symptoms. If the animal can't be observed (e.g. a wild dog) you should be treated.