The West Nile virus is one of the arboviruses (which are called that because they are borne (carried) by arthropods -- insects such as mosquitoes, ticks, and sandflies). It belongs to the same family (called "Flaviviridae") as the viruses that cause St. Louis encephalitis and yellow fever. West Nile virus, as the name implies, was first identified in Africa. It is also found in Asia, and has spread to Europe and North America, apparently through infected birds (the first known appearance of the virus was in dead birds in New York state).
The arboviruses are found in nature in birds and small animals,; insects carry the virus between these animals. People can be infected by being bitten by a carrier insect. The viruses seem not to be transmitted directly from person to person, as are other viruses such as influenza. However, recent cases seem to indicate that West Nile virus in particular can be transmitted from one person to another through transfused blood or transplanted organs or tissue. The incubation period appears to be 3-14 days after exposure.
Most (about 4 out of 5) people infected with West Nile virus have no symptoms whatever. I am not sure from my sources whether or not a single episode of infection makes you immune to the virus for life, but I would suspect that that is the case. (The vaccine for the yellow fever virus, which is in the same family as West Nile, is approved by international health authorities to protect a person for 10 years; some experts believe the yellow-fever vaccine gives lifetime immunity with one dose.) 1 out of 5 people infected with West Nile virus have a mild illness, with a suddenly-occurring fever that often comes along with headache, muscle aches, nausea, vomiting, loss of appetite, swollen lymph nodes, "malaise" (feeling lousy), a rash, and/or eye pain. The symptoms last about 3-6 days.
If these symptoms sound familiar -- especially if you've visited the Office before -- they should. Except for the rash and the eye pain, those are pretty much the symptoms of influenza -- and of a whole bunch of other common childhood viruses. One big difference, though, is that unlike influenza, which produces symptoms in just about everyone it infects, West Nile virus causes symptoms in only 20% of people who get it. Unfortunately, something similar can be said about the polio virus: 19 out of 20 people who were infected with the polio virus had few if any symptoms.
The problem with West Nile virus is that about 1 out of 150 people who catch it develop encephalitis (inflammation of brain tissue) or, less commonly, meningitis (inflammation of the membranes that surround the brain). Some infected patients have been reported to have developed "flaccid" paralysis of some of their muscles very similar to that seen in polio, which seems, like polio, to be caused by the West Nile virus affecting the spinal cord. There have been several reported deaths from West Nile virus in the United States, mainly from encephalitis; these seem to occur more often in older people.
So far, there is no specific treatment for a West Nile virus infection. As with most viruses, there is no antibiotic for West Nile virus. Severely affected patients usually need supportive care for the various problems that develop with infection.
There is as yet no vaccine for West Nile virus. I am not aware of how efforts to develop a vaccine are progressing. I personally suspect that one can be developed eventually. Since the virus is usually found in birds and other animals, I doubt that eradicating the virus (as we have done with smallpox and may be close to doing with polio) is even possible, but an effective vaccine would help reduce the number of severe cases.