The virus that inspired the whole zombie genre is, of course, rabies:
It’s a bullet shaped virus that can be transmitted through bodily fluids like saliva. Once in the body, it heads for the brain, causing swelling and a zombie-movie set of symptoms.
Actually, there are two kinds of rabies: dumb rabies and furious rabies. Dumb rabies is tougher to diagnose, since it comes on through slurred speech, loss of function, paralysis, coma, and death. Furious rabies, which comprises fifty percent of animal cases and two thirds of human cases, is where the horror movie stereotypes come from.
After an incubation period that could last anywhere from a week to several years, people with rabiees suddenly become antsy and hyperactive. They start becoming disoriented and lose lucidity. Eventually, they develop more aggressive symptoms. The mildest of these symptoms is simple irritability. People, metaphorically, snap at those around them. Because they’re hyperactive and restless, they tend to get annoyed with many people very quickly. As the disease progresses, though, they become more physically violent. A man in Mumbai became so violent that hospital personnel evacuated his room and eventually had to call the police and the fire brigade to pacify him enough for a sedative.
Patients become afraid of bright lights and moving air, so they try to hide in dark and confined places. They fall silent, in part because they become so afraid of the sound of their own voice, it’s impossible for them to speak. Most notoriously, patients with “furious rabies” develop strange appetites. Animals with rabies, although they can barely swallow water, have been seen to eat sticks and rocks. They also seem to undergo a compulsive need to bite. Scientists think that this is the disease trying, evolutionarily speaking, to strike out and get transmitted to a new host. This is the crux of the zombie mythology. A bite means a death of the self — loss of speech, coherence, lucidity, and ability to control aggressive impulses — and a rebirth as a silent, unresting zombie, endlessly driven to look for new people to bite.