Rabies is a viral disease that attacks the brain and spinal cord, or central nervous system (CNS). It's part of the Rhabdoviridae family of viruses, under the genus Lyssavirus. The virus itself, like all members of Rhabdoviridae, is shaped like a bullet. Upon entering the body, it makes its way to the spinal cord via the peripheral nervous system's afferent nerves (nerves that carry impulses toward the CNS). Once the virus gets into the spinal cord, it's quickly sent up to the brain, where it begins replicating itself inside the mind's nerve cells, destroying them in the process.
After it reaches the brain, the virus typically travels through the efferent nerves (nerves that carry impulses away from the CNS) to the salivary glands, which often causes increased salivation, or foaming at the mouth. It's important for the virus to do this, as this saliva is its principle method of transmission into new hosts. After hitting the salivary glands, the virus continues its way down throughout the rest of the body.
As you may have guessed from the way it operates, rabies is anything but an average disease. Though there are actually several different strains of the virus, there are only two real physical variations. The most common is the encephalitic, or "furious," form of rabies. This is the mad-dog, foaming-at-the-mouth version, usually highlighted by increased agitation and aggression, disorientation and hallucinations. This is the form most people imagine when they think of rabies. The other form, the paralytic or "dumb" form, is more peaceful, but no less deadly. With this version, the victim initially appears weary and lethargic.
Both of these forms take place during rabies' acute stage, the point at which the virus has successfully infiltrated the body, symptoms have occurred and all hope for recovery is gone. As the virus makes its way throughout the body, lethargy soon turns to partial or almost total paralysis, then to coma and death.
It's actually not uncommon for symptoms of both rabies forms to appear in a single case. If the person or animal doesn't die during the furious form, the virus will develop into the dumb form. Paralysis will occur, and the animal or person will slip into a coma and die, usually from respiratory paralysis.