Rabies is an ordinarily infectious disease of the central nervous system, caused by a virus and, as a rule, spread chiefly by domestic dogs and wild flesh-eating animals. Man and all warm-blooded animals are susceptible to rabies. The people of ancient Egypt, Greece and Rome ascribed rabies to evil spirits because ordinarily gently and friendly animals suddenly became vicious and violent without evident cause and, after a period of maniacal behaviour, became paralysed and died. Experiments carried out in Europe in the early nineteenth century of injecting saliva from a rabid dog into a normal dog proved that the disease was infectious. Preventive steps, such as the destruction of stray dogs, were taken and by 1826 the disease was permanently eliminated in Norway, Sweden and Denmark. Though urban centres on the continent of Europe were cleared several times during the nineteenth century, they soon became reinfected since rabies was uncontrolled among wild animals. During the early stages of the disease, a rabid animal is most dangerous because it appears normal and friendly, but it will bite at the slightest provocation. The virus is present in the sailvary glands(腺) and passes into the saliva so that the bite of the infected animal introduces the virus into a fresh wound. If no action is taken, the virus may become established in the central nervous system and finally attack the brain. The incubation(潜伏期) period varies from ten days to eight months or more, and the disease develops more quickly the nearer to the brain the wound is. Most infected dogs become restless, nervous, and irritable and vicious, then depressed and paralysed. With this type of rabies, the dog's death is inevitable and usually occurs within three to five days after the onset of the symptoms. Anti-rabies vaccine(疫苗) is widely used nowadays in two ways. Dogs may be given three-year protection against the disease by one powerful injection, while persons who have been bitten by rabid animals are given a course of daily injections over a week or ten days. The mortality rate from all types of bites from rabid animals has dropped from 9% to 0.5%. In rare cases, the vaccine will not prevent rabies in human beings because the virus produces the disease before the person's body has time to build up enough resistance. Because of this, immediate vaccination is essential for anyone bitten by an animal observed acting strangely and the animal should be captured circumspectly, and examined professionally or destroyed. Rabies is______.