Rabies: It is the most fatal virus known to mammals. The rabies virus attacks the brain and central nervous system, and is transmitted through saliva from the bite of an infected animal. This is also known as a zoonotic disease, meaning it can be passed on to humans. Symptoms usually develop between 20 and 60 days after exposure. Rabid animals may become aggressive, combative, and highly sensitive to touch and other kinds of stimulation. This is the "furious" form of rabies traditionally associated with mad dogs. There is also a "dumb" form of the disease, in which the animal is lethargic, weak in one or more limbs, and unable to raise its head or make sounds because its throat and neck muscles are paralyzed. In both kinds of animal rabies, death occurs a few days after symptoms appear, usually from respiratory failure. Immunization should be done at 3 months of age, and then repeated yearly to protect against this deadly virus.