Risk Factors

Encephalitis is a rare disease. It is extremely uncommon in the U.S., even for people in the risk groups discussed below. Many people fall into the following categories and very few of them will ever contract encephalitis.

Age

Encephalitis can occur at any age; increased age-associated risks depend on the type of encephalitis virus. Newborn infants are particularly at risk for herpes virus. For arboviruses, infants are most vulnerable to Western equine encephalitis; older children and teenagers are more susceptible to Eastern equine and La Crosse encephalitis; older and elderly adults are at higher risk for Eastern equine, St. Louis, and West Nile encephalitis.

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Immunocompromised Patients

Patients whose immune systems are compromised by conditions such as HIV-AIDS, cancer therapies, or organ transplantation are more susceptible than other individuals to any form of encephalitis. Of particular concern are varicella and cytomegalovirus encephalitis which tend to be more common and deadly in these patients than in the normal population.

Risk Factors for Arboviruses

U.S. Geographic Regions. The primary risk factor for arbovirus encephalitis is living in areas of possible exposure to virus-carrying mosquitoes. Most viral outbreaks occur in rural or farming areas, but they can also occur in cities. While some forms of arbovirus encephalitis are limited to specific geographical regions, the West Nile virus has become endemic throughout the mainland United States. (See Common Forms of Mosquito-Borne Encephalitis table for more detailed regional information.)

Seasonal Risks. Transmission of arboviruses correlates with the mosquito season and is highest during the months of July through September (late summer through early fall). The ideal conditions for mosquito breeding are a wet spring followed by a hot, dry summer.

Risk Factors for Herpes Viruses

It is a very rare individual anywhere in the world who has not been infected with at least one of the herpes viruses. Most of these viruses are easily transmitted in body fluids, including saliva or in droplets after people exhale or sneeze. Infants can contract herpes simplex virus from an infected mother during delivery, which can have very serious consequences. [For detailed information, see In-Depth Report #52: Herpes Simplex.]