health

The exact number of cases of encephalitis in the United States in an
average year is difficult to estimate because mild cases may not be diagnosed,
and the reporting of severe cases is not standardized. Between
1,500 and 3,000 cases of viral encephalitis are reported to the Centers for
Disease Control and Prevention (CDC) each year, but most doctors
think that the actual number of cases is higher. According to the CDC,
it costs about $150 million each year to control the mosquitoes that
spread some types of viral encephalitis.

Some statistics for specific types of viral encephalitis are as follows:

• Japanese encephalitis: 30,000–50,000 cases each year in Asia;
  about one case per year in Americans traveling in Asia.
  
• Eastern equine encephalitis, found in the eastern United States:
  five cases per year on average.
  
• La Crosse encephalitis, found in California and the Great Lakes region:
  seventy cases per year on average.
• West Nile virus: about 1,200 cases per year in the United States.
• Herpes simplex encephalitis, caused by the virus that causes cold sores:
  about one person in every 250,000 per year in the United States.
• Encephalitis following measles: about one case per 1,000 infected persons.
• Encephalitis following chickenpox: one case in every 2,000 infected persons.
• Encephalitis following rabies: one tothree cases in the United States each year