Mild signs and symptoms of West Nile virus infection usually resolve on their own. Strong signs and symptoms, however, such as a severe headache, fever, disorientation, or abrupt weakness, necessitate emergency medical intervention. These symptoms are most often associated with older adults, but anyone can become ill. There is no evidence that suggests that children under the age of 18 cannot get West Nile virus. West Nile virus disease remains endemic in many parts of the United States. Because infected mosquitoes transmit the virus to humans only during certain months of the year, the number of reported cases varies from year to year. However, because infection always results in symptoms, the number of undetected cases likely exceeds reported ones. The virus was first identified in 1999 in New York City. Since then, increased incidence of West Nile virus has been observed throughout the United States. Although most people who are infected with West Nile virus do not develop serious illness, some develop mild flu-like symptoms after being bitten by an insect infected with West Nile virus. Others may experience more severe neurological problems including muscle weakness, paralysis, and death. Infection of pregnant women has resulted in births having low birth weights and premature infants. In addition, one in 150 infections leads to severe brain damage or death. There is no treatment for West Nile virus infection.