Encephalitis is an acute infection of the brain characterized by fever, headache, and an altered state of consciousness, with or without seizures. Most cases of encephalitis are viral. They may also be focal (limited to a single location) or generalized (diffuse throughout the brain). Each year there are several thousand cases reported but, according to the NINDS, there are probably many more cases with minimal to mild symptoms that occur but are not documented.

Viral encephalitis may be caused by a variety of viruses including herpes simplex virus, enteroviruses, the rabies virus (from an animal bite), or arboviruses – those spread primarily by infected mosquitoes but also by a few ticks.

Humans are not the preferred or primary host of the arboviruses. Most people who are infected have mild to moderate symptoms. Only a very small percentage of people develop encephalitis. In the United States, West Nile Virus is the most common cause of arbovirus encephalitis. Other more rare U.S. arboviruses are distributed geographically. Throughout the world, different types of arbovirus-related encephalitis may predominate. Types of encephalitis caused by arboviruses include:

• West Nile encephalitis—about one in 150 people infected with West Nile Virus will develop severe illness. In 2007, 1,204 cases of WNV meningitis/encephalitis were reported to the Centers for Disease Control and Prevention (CDC).
• Western equine encephalitis – Per the CDC, about 3% of those affected die; more severe in infants than adults.
• Eastern equine encephalitis – Per the NINDS, there are fewer than 10 cases a year. The CDC estimates that about a third will die and many survivors will suffer brain damage.
• LaCrosse encephalitis – Found in the upper Midwest; per the CDC, about 75 cases a year, mostly in children less than 16 years of age.
• St. Louis encephalitis – Per the CDC, there are an average of 193 cases a year; most severe in the elderly.

Other arboviruses that may be seen internationally include:

• Japanese encephalitis – Per the CDC, worldwide over 45,000 cases reported each year.
• Venezuelan equine encephalitis – Very rare in U.S. It has killed thousands of people in South American epidemics.

Viral encephalitis may also be seen as a secondary condition that occurs a few weeks after a viral illness.

Bacterial, fungal, and parasitic encephalitis are very rare. Bacterial meningoencephalitis may develop from the bacteria that cause meningitis. Tick-transmitted Lyme disease may cause bacterial encephalitis. Toxoplasma gondii, a parasite associated with cats, can cause parasitic encephalitis in some people with compromised immune systems. Other bacteria, fungi, and parasites can occasionally cause encephalitis.