Encephalitis is a rare but potentially life-threatening inflammation of the brain that can occur in people of all ages. The most common cause of encephalitis is infection by a virus. In very rare cases, encephalitis can also be caused by bacterial infection, parasites, or complications from other infectious diseases. This report focuses on viral encephalitis.

Encephalitis occurs in two forms ? a primary form and a secondary form. Primary encephalitis involves direct viral infection of your brain and spinal cord. In secondary encephalitis, a viral infection first occurs elsewhere in your body and then travels to your brain.

Mosquitoes are responsible for the transmission of encephalitis from small creatures, usually birds and rodents, to humans. Birds who live near bodies of standing water, such as freshwater swamps, are susceptible to infection with an encephalitis virus. When a bird is infected with encephalitis, it carries high levels of the virus in its blood for a short time before recovering from the infection and developing immunity to the disease. If a mosquito feeds on an infected bird, the mosquito will become a lifelong carrier of the disease.

One of the most dangerous and most common causes of encephalitis is the herpes simplex virus (HSV). HSV is the same virus that causes cold sores around the mouth, but when it attacks the brain it may occasionally be fatal. Fortunately, HSV encephalitis is very rare.

West Nile virus is carried by mosquitoes and is most common during the summer and early fall. The best way to prevent becoming infected with West Nile virus is to avoid being bitten by a mosquito. Use insect repellant when you go outside, especially during the peak mosquito hours of dusk and dawn. Remove mosquito-breeding environments (such as standing water in flower pots) from your property. Scientists are testing several different vaccines to protect against West Nile virus, but it will be many years before they are commercially available.

Coma is a common symptom in patients with severe encephalitis, but does not necessarily predict a fatal or severe outcome. In one study of Eastern equine encephalitis, some survivors averaged 5 days in a coma and had no or only mild to moderate complications afterward. One patient was in a coma for 9 days and had only mild complications afterward.

Encephalitis can develop shortly after a virus is introduced by the bite of an infected insect, or it can be caused by a virus that has been latent in the body. Viruses are simple but powerful infective agents: The virus infects the host by penetrating a cell wall and ejecting its genetic material (its DNA or RNA) into the cell. The viral DNA or RNA takes control of important process in the cell, which redirects to produce more viruses.

Viral encephalitis is the inflammation (damage to cells) of brain tissue that may result from infection with any of a number of viruses. Viruses are extremely small infectious agents that are different from bacteria, cannot live outside of cells, and cannot be treated with antibiotic drugs. Herpes simplex virus type 1 is the most common cause of encephalitis in the United States.