This article was written specifically for House Eclipse by Dr. Goldmakher, please do not reproduce this article elsewhere without permission.
The purpose of this article is to expand upon and clarify the brief article on rabies posted by Ransim.
Rabies is a disease caused by a virus which infects the nervous system. The infection is usually transmitted through an animal bite. Dogs (but not cats) can transmit the disease, but, SPECIAL NOTE TO THE VAMPIRE COMMUNITY, bats are particularly notorious for transmitting it. This means that if you're out hiking or are in your house and you encounter a bat, no matter how adorable you think it is DO NOT TOUCH IT unless you are able and willing to go to the nearest clinic and get your shots done within a day or two. Skunks, racoons, foxes, and coyotes are the other animals that have been documented rabies vectors in North America (where I am assuming most readers of this are located).
The virus (a type called a Lyssavirus) is a scary critter which enters your peripheral nerves by binding to receptors for the neurotransmitter acetylcholine. It then travels up the sensory nerves into the brain. The time it takes to travel is variable, and accounts for the incubation period seen between the bite and the onset of symptoms. The incubation period (bite to first symptom) ranges from 4 days to at least 19 years, but in about 75% of cases it is between 20 and 90 days.
"After 1-7 days of generally non-specific symptoms, features of either furious or paralytic (dumb) rabies develop.
The pathognomonic symptom of furious rabies is hydrophobia or aerophobia. Jerky and violent contractions of the diaphragm and accessory muscles of inspiration, are associated with an inexplicable terror. This reflex can be provoked by attempts to drink liquid or by a draught of air on the skin. The hydrophobic spasm may end with the patient in opisthotonos, having generalized convulsions and in cardiac or respiratory arrest. Other features of furious rabies are periodic excitement, sometimes with hallucinations or aggression, interspersed with lucid intervals, tachycardia and other arrhythmias, hypersalivation, lacrimation, sweating, and fluctuating temperature and blood pressure. All these features are suggestive of stimulation of the autonomic nervous system as seen in severe tetanus. Conventional neurologic examination may prove surprisingly normal but meningism, cranial nerve lesions, upper motor neuron lesions, fasciculation, and involuntary movements are sometimes detected. Without intensive care, patients usually die, often during a hydrophobic spasm, in the first few days of their encephalitic symptoms.
Paralytic or dumb rabies is probably underdiagnosed but was particularly associated with an epidemic of rabies transmitted by vampire bats in Trinidad in the 1920s and 1930s. After the prodromal symptoms described above, together with fever and headache, local paresthesia and flaccid paralysis develop, usually in the bitten limb, and ascend symmetrically or asymmetrically. There is accompanying pain and fasciculation in the affected muscles and mild objective sensory disturbances. Death follows paralysis of the muscles of deglutition and respiration; there is usually no evidence of hydrophobia or aerophobia. Some patients have survived for as long as 1 month, even without intensive care."
To translate, summarize, and add a couple of points, the classic or "furious" form of this disease causes bizarre behaviors, in which hallucinations, hyperactivity, and aggression (specifically sometimes including a tendency to bite) are prominent. Hypersalivation ("foaming at the mouth") is common, and the saliva can be tinged with blood, usually because the patient has bitten his/her tongue or cheek and is bleeding into their mouth.
Apparently, there was a major epidemic of rabies among wild and domestic animals in Hungary in the 1720s. Based on this, the timing of the emergence of vampire legends in Europe, and on the similarities between aspects of the legends and the clinical manifestations of classic rabies, there has been a proposal by a Spanish physician that the original legends were inspired by a bunch of people getting infected in one area in a short period of time.
If you have enough interest in this to read the full article, here is the reference (you can order the article through your library, or go to any medical center library and they'll have it in their collection):
Gomez-Alonso, J. Rabies: A possible explanation for the vampire legend. Neurology 51(3): 856-859. September 1998