Encephalitis is a very rare inflammation of the white and gray matter of the brain. While technically referring to any brain inflammation, the term encephalitis is most commonly used to describe inflammation caused by a viral infection.

Children are among the populations most affected by encephalitis. Children with encephalitis are likely to have problems with brain and spinal cord function. Changes in the child’s neurologic condition cause mental confusion, seizures and other symptoms.

Mosquito- and tick-borne viruses (arboviruses) play a significant role in the spread of encephalitis worldwide. Birds and other small mammals serve as hosts to the disease. A mosquito feeding on an infected bird becomes an encephalitis vector (carrier) for the remainder of its life. The virus is then passed by the mosquito to other animals it feeds on, which, in turn, infect other mosquitoes feeding on that animal.

Arboviruses are of growing concern in light of continued global population growth, concerns of bioterrorism, social and environmental instability in underdeveloped countries and regions as well as global warming.

Encephalitis is most hazardous to infants, the elderly, individuals with compromised or weakened immune systems (e.g., cancer or HIV/AIDS patients) or people who reside in areas with high mosquito populations. Although children younger than age 1 and adults over age 55 are at the greatest risk of death, encephalitis can be potentially severe or even fatal to people of all ages.

The viral infection can occur either directly in the brain and spinal cord (primary encephalitis) or spread to these areas from elsewhere in the body (secondary or post-infectious encephalitis). It is also possible, though uncommon, to contract encephalitis through other organisms, such as bacteria, fungi or parasites.

A swollen or irritated brain will not maintain normal blood flow, usually causing ailments such as fever, headache or confusion. Frequently, the root cause of encephalitis can also agitate or inflame frail tissue structures lining the brain and spinal cord (meninges). When the meninges become inflamed, this condition is known as meningitis. It is not uncommon for encephalitis and meningitis to occur simultaneously.

When encephalitis (inflammation of brain tissue) and meningitis (inflammation of the membranes lining the brain) occur in combination, the condition is called meningoencephalitis. It is not uncommon for untreated meningitis to cause encephalitis, because in a number of cases infection spreads from the meninges to the brain. When the spinal cord is also affected, the term encephalomyelitis may be used.